2004 GMC Sierra 4WD Problem

51,000 miles. I drive around all the time with 2WD engaged. Recently, noticed when I cranked up, the " auto 4wd " button lit up and "2wd " was not. I pushed 2 wd button and everthing okay. Here`s the new problem.
Sometimes when I am driving, usually slow and in a turn, the truck jumps into 4WD by itself. Engine revs up, hard to turn, etc. even on pavement.
I put it in neutral and press the auto 4wd button and wait till it lights. Then I push the 2WD button again and it goes back to 2WD.
I just had transmission serviced by very trusted shop, but they can not find my problem. Seems like no codes are stored and no idiot lights are lit up. Can it be my Encoder ? Solenoid in the encoder, or dashboard switch ?
Help please.

A bad instrument panel sector switch was a common problem. They had to replace mine on my 2004 GMC under warranty. Very common, some more of the Delphi junk that GM got stuck with. I think they are around $25-$40.

Finally got my problem fixed. Seems it was the ole Encoder problem with a side dish of computer module ( which has to be reprogrammed at Chevy/ GMC in my area. ) The first day the shop had it , it jumped in 4 low and would not disengage. The second day they corrected the problem and pryed abour $1300 bucks from my wallet. This is the second General Motors Vehicle I have owned and taken baby care of it and the drivetrain messed up around 50,000 miles. Seems like I would learn.