2000yukondanali i replaced switch for 4x4 and service 4x4 light went out for 1 day now switch lights work service 4x4 still on and wont shift to 4x4 hi or 4x4 low

The Front Axle Actuator is probably your problem. I have replaced my Actuator TWICE in the last 7 years.... The Reason for the Service 4WD was the Failed Attempt. If you are able to test the Power going to the Actuator and there is none, then you have one of 3 switches that went bad but Chevrolet/GM is known for having the Defective Actuators. Aftermarket Actuators are available.

Output speed sensor needs replacing

is it possible that the switch is not plugged in all the way,? i know this is a stupid question, but you said the radio was ripped out as maybe stolen, now the other day i pulled the radio out of mine as the display stopped illuminating and the same thing happened. the 4x4 switch worked fine before, i started to look around wiggle things when the plug fell onto my hand, i plugged it in and it started to work again,ok fixed! next day, now the 4hi and 4lo lights flash three times and stop then a few min later flash three times and stop, and wont switch into 4x4 lo or hi stuck in auto 4x4

Your brake an CV joint issue can't an won't cause your 4X4 problem . The 4x4 system has a control module which supply's B+ voltage to the switch . Checking for codes in the 4X4 control module should have been the first step . Your best bet , take it to a qualified repair shop . You don't have the knowledge or test equipment to fix this.
SERVICE indicator (4WD/AWD) Lamp
The SERVICE indicator (4WD/AWD) lamp is an integral part of the cluster and cannot be serviced separately. This lamp is used to inform the driver of the vehicle of malfunctions within the automatic transfer case (ATC) system. The SERVICE indicator (4WD/AWD) lamp is controlled by the transfer case shift control module via CKT 1567.

Transfer Case Shift Control Module
The transfer case shift control module uses the VIN information for calculations that are required for the different calibrations used based on axle ratio, transmission, tire size, and engine. The system does not know which calibration to use without this information.
The direct battery supply line (CKT 1640) provides the power to the module and motor circuitry.
Transfer Case Encoder Motor
The transfer case encoder motor consists of a permanent magnet (PM) DC motor and gear reduction assembly. It is located on the left hand side (drivers side) of the transfer case. When activated it turns the sector shaft of the transfer case (clockwise or counter clockwise) to shift the transfer case. The encoder motor is controlled with a pulse width modulated (PWM) circuit within the transfer case shift control module. This circuit consists of a driver on both the Motor A and Motor B circuits. The encoder motor is bi-directional to allow the motor to shift the transfer case from 2HI or 4HI to NEUTRAL and 4LO positions.
Do you know what DTC'S are ?
DTC B2725 ATC Mode Switch Circuit Malfunction
DTC C0300 Rear Speed Sensor Malfunction
DTC C0305 Front Speed Sensor Malfunction
DTC C0308 Motor A/B Circuit Low
DTC C0309 Motor A/B Circuit High
DTC C0310 Motor A/B Circuit Open
DTC C0315 Motor Ground Circuit Open
DTC C0323 T-Case Lock Circuit Low
DTC C0324 T-Case Lock Circuit High
DTC C0327 Encoder Circuit Malfunction
DTC C0367 Front Axle Control Circuit High
DTC C0374 General System Malfunction
DTC C0376 Front/Rear Shaft Speed Mismatch
DTC C0387 Unable to Perform Shift
DTC C0550 ECU Malfunction
DTC C0611 VIN Information Error

drop off @ dealer let them fix it over my head

Sounds like the PRNDL switch on the side of the transmission. Be sure and clear the code before trying to start after replacing it.

You may have a bad electric shift motor on the tranfercase and the push button feature in the dash may be bad.

Hi my name is Juan and I do own a trailblazer with the same year 2002 and I read the problem you have let me try to help you Have you try to shift to 4hi with your transmission in neutral ? the transmission needs to be in neutral in order to shift the transfer case to 4hi and the vehicle needs to be completely stooped also is your service 4wd light on?? if the light is on your transfer case needs service and it can be as simple as the switch inside your car or a solenoid that has not been detected by the computer

I had the same problem. Went out to go somewhere and it wouldn't shift out of park. It is really the shift solenoid located under the gear box. It doesn't remotely sound like brake light switch, common sense should tell you this is one of the last things it could be. I had 'backyard mechanics' look at it and everyone had a different solution. Then I actually went to a shop, where in 5 mins, he had diagnosed the problem. The hardest part is ordering part. Its skyhigh @ dealership or wait 3 days for auto parts store to order. Good luck1

i had the same problem and it ended up being short in the front axles where the actuator at or the switch on the transfer case under body where the shift lever at