I don't know how to post stuff
But I have a 2002 gmc 2500 6.0 it says it goes into 4x4 and the light says its in but it doesent work, alsO the service 4 wheel drive light is on

If you have a service light on, I suggest you take it to a shop that can read the codes for your 4x4 system. We really cannot help if we don't know what those codes are.

you could put the truck up on jack stands and see what is working. if the front driveshaft is turning but the front isnt engaging something is usually wrong with the actuator. you can take it out and see is it goes back and forth, if not and you get voltage the actuator itself is the issue

Agreed on the Jackstands idea. It is a free thing that would not take you very long. The other option is to get the 4wd codes scanned, which is likely going to cost diagnostic time at the dealership.

Yeah did the jack stand, it says it goes into 4 high and 4 low but doesent engage, you can hear the actuator and I t

?
does the front driveshaft spin?

sorry i never finished my sentence, basically my 4x4 never really worked, the auto worked all the time, 4 high it would only go in sometimes, and 4 low the dash would just basically tick then the whole button system would shut down and i would have to restart the truck to get the buttons lights to turn back on. Even when i took the actuator off and put it into 4 the pin would engade (lights on the buttons would flash) the pin would retract and it would go back into 2 wheel.... recently i went 4x4ing and just used the auto then blew a ecu fuse or something, replaced the fuze now it goes into 4 hi and 4 low but it sounds different then before but it does not actually engage the front wheels just the light goes on... This was 2 days ago, should i take the actuator off again and see if the pin moves now as it sounds different?, and im not sure on the drive shafts i will jack it up and check tonight and post back on my info