Quote Originally Posted by DylanL View Post
I know this is a pretty old thread but i have a 99 gmc sierra 5.3L 4l60e tranny has 250k miles. Tranny has had a semi hard down shift into first for a while now but it got way worse today. Drove about 5 miles and completely lost all gears (P,R,N,D etc.) along with loud grinding and popping sounds but only in 2wd. i can put it in 4 wheel high and it drives decent, minimal slipping and not really any grinding. Im thinking its the TC but wanted to see what you guys think. A buddy of mine was saying it may be my rear end.
I would look to get another vehicle or try to find some scrap yard replacements if you are looking to keep it. to rebuild the transmission and /or transfercase will cost a couple of thousand. too much for a vehicle in this condition.

check all drivetrain fluids for condition.. could be the tranny sunshell has cracked Dexron III eats away at the internal components with high mileage on the tranny fluid.