It run good but transmission takes long to shift

Check the
Vehicle Speed Sensor The vehicle speed sensor (VSS) (Figure 43) is a variable reluctance or Hall-effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling. That same VSS signal is use for the tachometer as well !

I'm sure sure its the speed sensor on the transmission it may be just a loose connection or a bad sensor.

part of a clutch job is removing the gearlever make sure the lever is secured properly and selects the gear correctly.
Orherwise your symptoms sound like a collapsed pressure plate or broken release bearing either defective part or something done wrong during installation of clutch

I want you to check something. there is 2 cables on the throttle body. 1 cable is to the gas pedal and the other is your a kick down cable . the kick down down in the front of the transmission if you look under the battery it does in the front of the transmission there. see if the kick down moves easy if you open the throttle body by hand. the problem with those transmission is it relis on the cable the shift the gears up . if you steped on the gas pedal that cable is to pull levers inside to up shift. there is not shift solenoids in those transmission. see many in my shop

Check the Vehicle Velocity Sensor (V V S)-(Speed Sensor) or the wire harness to the VVS. IF the ECM is not getting a signal from the V V S, the PCM can not give the signal to switch gears. The V V S is on the drivers side of the end ot the transmission that is held on by one bolt. Best thing to do is to have the truck scanned at Auto Zone for a DTC code and to confirm that the V V S has gone bad and they can give you the step by step instruction to replacing the sensor and takes about 15 min to do. Good luck and keep me posted. Keep an eye on the transmission fluid, make sure you don't run the transmission to hard and burn it out over a $40.00 sensor which is better then $1,800 for a transmission replacement. 

Your going to have to get the ECM scanned, Auto Zone scan's for free and most garage now will scan for $10.00 to $15.00 since they all have the scanners now and need the business.
You could have a Vehicle Velocity Sensor (V V S)  going bad or the wire harness it self. The V.V.S tells the ECM how fast the drive shaft is rotating and calculate's what gear it should be in relative with the engine speed and throttle position, and transmission fluid pressure.
Regardless you need the Blazer scanned to pin point why your transmission is shifting the way it is. Could be as simple as time for transmission oil and filter change for $125.00 or a loose wire harness.
Check your transmission fluid and try not to run the blazer to hard, to replace the transmission start at $1,900.00 for a rebuilt.
Good luck and hope this helps. Keep me posted

You have two sensor in the transmisson which is the in put and out put speed sensor, and is located on the left side of the transmission. 
They are the primary input to the transmission control module which control your shift point and speedometer.
They are easy to replace and only held on by one bolt. Disconnect the wire connector and then the bolt. 
Replaceing is in revers and replace any fluid lost. If you could get it to the parts store and have the truck scaned, they could tell if you need both sensor or only one. Before you go out to the parts store, check the wire connections to see if there lose or broken which may be the cause of your trasmission problem.
Good luck and hope this helps and saves you some money

the flashing O/D light usually means you have a bad speed sensor that sends speed info to the engine & trans computer.

Vehicle Speed Sensor The vehicle speed sensor (VSS) (Figure 42) is a variable reluctance or Hall-effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling.
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Vehicle Speed Sensor The vehicle speed sensor (VSS) (Figure 42) is a variable reluctance or Hall-effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling.

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Some trans use an OSS, Output Shaft Sensor instead of a VSS. Depends on which trans you have, 4R100 or 4R70W

Transmission Control Switch (TCS) and Transmission Control Indicator Lamp (TCIL)
The transmission control switch (TCS) is a momentary contact switch. When the switch is pressed, a signal is sent to the powertrain control module (PCM) to allow automatic shifts from first through fourth gears or first through third gears only. The PCM energizes the transmission control indicator lamp (TCIL) when the switch is off.

The TCIL indicates overdrive cancel mode activated (lamp on) and electronic pressure control (EPC) circuit shorted (lamp flashing) or monitored sensor failure.

Output Shaft Speed (OSS) Sensor
The output shaft speed (OSS) sensor is a magnetic pickup, located at the output shaft ring gear, that sends a signal to the powertrain control module (PCM) to indicate transmission output shaft speed. The OSS is used for torque converter clutch control, shift scheduling and to determine electronic pressure control.

I have the same problem also. Hope someone has the answer.