Ok I just swapped the transfer case on my 1999 tahoe and When I'm not moving I can push all the button like 4hi,4lo and auto but if I leave it in 4wd and start driving the truck will stay in 4hi or whatever I chose and won't switch back to 2wd.But if I turn the truck off I can go back into 2wd. I disconected the battery and pulled the fuse do think it shoud be scanned or reprogrammed or something.

What transfer case does it have?
Are all 4 tires matched up?
Whe stopped in park or neutral will it shift into and out of 4W and go into 4WL and back into 4WH and then to neutral or 2W.
Key on for a bulb check does the shift buttons alll light up and go out or does any stay on or blink?
Does the shift buttons have the auto button?

Yes when I'm in park or neutral the lights blink and stay on and I feel them kick in.But like I said witch everone I take off in thats the one it will get stuck in and the services 4wd light comes until I shut it off.

Do you have the auto button on shift buttons?

Yes I have the auto button cuz I have the 246gm Transfer case.

Yes I have the auto button cuz I have the 246gm Transfer case.
On that one you will need to get it ob a transfer case scanner and check for codes and do the shifting test that will point out the problem.
Post back any codes.
Also that ones needs to be serviced a reg mileage with the GM blue transfer case fluid.
If it has not been service that can be part of rhe problem.
When was the last service on it?