96' tahoe-4wd problems

hello all, I am new to the forum and not a seasoned mechanic. I have recently had problems with my 4wd switching back into 2wd. I have automatic switch and it goes into 4wd fine but does not want to switch back imeadetly. The 2wd button is engaged and lite up, but transmission is still in 4wd. Eventually afer a couple of miles of driving a loud "thud" is heard and then it switches back to 2wd. Is this a transfer case problem? Any suggessions are greatly appreiated. thanx

Try backing up a little after putting it back in 2wd... sometimes you'll develop a torsion on the gears in the transfer case (do to off-road surfaces, tires, etc.) - a binding that won't allow it to shift out of 4wd. Backing up a little will relieve that torsion and you'll be good to go.

Bob