Where is the transmission speed sensor located on a 2007 4x4 trailblazer, will it cause the transmission not to shift into 3rd gear?

Sounds like a speed sensor has failed. Could also be that the transmission fuse has blown. Check fuses and get codes cleared. Other things, that will affect the transmission shift points, would be the manifold absolute pressure MAP sensor, throttle position sensor, PRNDL switch (shifter location switch). Probably just a power issue.

Sounds like a broken transmission or motor mount and is putting shift cable in a bind.

Check the linkage under the stick console. May be lose or broken.

When did you last replace trans filter and fluid? Dirt may be causing shift valve to stick.

P0700 is a informative transmission code, about some problem there; the P0733 and P0734 is the specific problem there.
I would first try changing the fluid and filter. Although the fluid may look like it is brand new, with 100K on the fluid, I can assure you that even if those miles were all highway at steady speed and no load that the fluid has deteriorated and no longer lubricating the internal parts properly and that there is some debris in the ******.If codes return and shifting problems are still present, there is definite internal failure, possibly the pump or in the clutches.
DTC P0733The powertrain control module (PCM) uses the crankshaft position (CKP) sensor, input speed sensor (ISS) and output speed sensor (OSS) to determine engine speed, input shaft speed and output shaft speed. These sensor readings are used to determine the current gear ratio and RPM slip across the torque converter. The PCM uses this information from the sensors to determine if the commanded gear matches the actual gear. During normal driving conditions, the PCM will adapt main line pressure via the pressure control (PC) solenoid to prevent slipping and harsh shifts. Shift solenoid 3 is used to apply or release third gear clutch. The PCM controls 3rd gear by controlling the ground for shift solenoid 3. The PCM will turn the solenoid OFF, open the driver circuit, when 3rd gear is commanded On to apply 3rd clutch. Shift solenoid 3 is supplied ignition voltage from the fuel pump relay whenever the PCM commands the fuel pump relay ON. DTC P0733 sets when 3rd clutch still slips after the PCM has commanded high main line pressure.DTC Parameters
DTC P0733 will set if the PCM detects an incorrect actual 3rd gear ratio when 3rd gear is commanded On when:- The condition exists for longer than 3 seconds (2001)*- The condition exists for longer than 3 seconds, 1 minute is allowed to elapse, then condition exists again for longer than 3 seconds (2000)- The high main line pressure has been commanded- The transaxle is in a forward gear- The input speed is greater than 224 RPM- The vehicle speed is greater than 5 km/h (3 mph)- The speed noise parameter = NO- The steady state ratio fail counter equals 255 for an in-gear failure, or the shift fail counter is greater than 100 on a downshift or upshift to 3rd gear (128=0 or normal operation)DTC P0733 diagnostic runs continuously once the above conditions have been met.
DTC P0733 is a type A DTC for 2000, type B for 2001.**Applicable to vehicles built after June 2000.
Diagnostic AidsImportant: If DTCs P0745, P0746, P0756, P0758, P0761, P0763, P0766, P0768 or P0789 is set, diagnose that DTC first. A diagnosed PC solenoid, shift solenoid 2, 3 or 4 low/high circuit fault DTC may have caused DTC P0733 to set.Important: If a shift solenoid performance DTC P0783, 4th gear stuck ON, is set, diagnose that DTC first. A 4th gear stuck ON may cause no 3rd gear, no 2nd gear, or TCC stuck Off.
Refer to Pressure Ports - Air Check and Parts Description for pressure apply points and for component/location description.
Possible causes of no 3rd gear are listed in the flowchart on the succeeding page.
Verify TRANSMISSION ISS and TRANSMISSION OSS read correctly on Scan tool.
Make sure the pressure control, shift 2, shift 3 and shift 4 solenoids are not intermittently sticking. A sticking shift solenoid 4 may cause 4th gear to be applied from an initial start resulting in no 3rd gear. Audible sound should be similar for all solenoids. A muffled tone indicates the need for replacement.
Check this pinout HERE
P0734 refer at ratio 4 gear, and the rocedure is the same.
Hope helps.

Is your code reader capable of reading U, C, and B codes? The speedometer doesn't get it's speed reading from the wheel sensors. It gets it from the transmission. Not shifting past 2nd gear means your transmission is in "limp in" mode. It prevents shifting to avoid further damage. I see a transmission repair shop in your future.

P0700 is a informative transmission code, about some problem there; the P0733 and P0734 is the specific problem there.
I would first try changing the fluid and filter. Although the fluid may look like it is brand new, with 100K on the fluid, I can assure you that even if those miles were all highway at steady speed and no load that the fluid has deteriorated and no longer lubricating the internal parts properly and that there is some debris in the ******.If codes return and shifting problems are still present, there is definite internal failure, possibly the pump or in the clutches.
DTC P0733The powertrain control module (PCM) uses the crankshaft position (CKP) sensor, input speed sensor (ISS) and output speed sensor (OSS) to determine engine speed, input shaft speed and output shaft speed. These sensor readings are used to determine the current gear ratio and RPM slip across the torque converter. The PCM uses this information from the sensors to determine if the commanded gear matches the actual gear. During normal driving conditions, the PCM will adapt main line pressure via the pressure control (PC) solenoid to prevent slipping and harsh shifts. Shift solenoid 3 is used to apply or release third gear clutch. The PCM controls 3rd gear by controlling the ground for shift solenoid 3. The PCM will turn the solenoid OFF, open the driver circuit, when 3rd gear is commanded On to apply 3rd clutch. Shift solenoid 3 is supplied ignition voltage from the fuel pump relay whenever the PCM commands the fuel pump relay ON. DTC P0733 sets when 3rd clutch still slips after the PCM has commanded high main line pressure.DTC Parameters
DTC P0733 will set if the PCM detects an incorrect actual 3rd gear ratio when 3rd gear is commanded On when:- The condition exists for longer than 3 seconds (2001)*- The condition exists for longer than 3 seconds, 1 minute is allowed to elapse, then condition exists again for longer than 3 seconds (2000)- The high main line pressure has been commanded- The transaxle is in a forward gear- The input speed is greater than 224 RPM- The vehicle speed is greater than 5 km/h (3 mph)- The speed noise parameter = NO- The steady state ratio fail counter equals 255 for an in-gear failure, or the shift fail counter is greater than 100 on a downshift or upshift to 3rd gear (128=0 or normal operation)DTC P0733 diagnostic runs continuously once the above conditions have been met.
DTC P0733 is a type A DTC for 2000, type B for 2001.**Applicable to vehicles built after June 2000.
Diagnostic AidsImportant: If DTCs P0745, P0746, P0756, P0758, P0761, P0763, P0766, P0768 or P0789 is set, diagnose that DTC first. A diagnosed PC solenoid, shift solenoid 2, 3 or 4 low/high circuit fault DTC may have caused DTC P0733 to set.Important: If a shift solenoid performance DTC P0783, 4th gear stuck ON, is set, diagnose that DTC first. A 4th gear stuck ON may cause no 3rd gear, no 2nd gear, or TCC stuck Off.
Refer to Pressure Ports - Air Check and Parts Description for pressure apply points and for component/location description.
Possible causes of no 3rd gear are listed in the flowchart on the succeeding page.
Verify TRANSMISSION ISS and TRANSMISSION OSS read correctly on Scan tool.
Make sure the pressure control, shift 2, shift 3 and shift 4 solenoids are not intermittently sticking. A sticking shift solenoid 4 may cause 4th gear to be applied from an initial start resulting in no 3rd gear. Audible sound should be similar for all solenoids. A muffled tone indicates the need for replacement.
Check this pinout HERE
P0734 refer at ratio 4 gear, and the rocedure is the same.
Hope helps.

Check your transmission fluid level and/or have the fluid changed. If problem persist have a transmission professional at a dealership or transmission shop service it.

sounds like trans speed sensor. Need to have the sensor checked and replaced.