Truck Forums - Differential stays locked in 4wd...99 Silverado

Here's some info on the truck...

My dad bought it new in 99, it has 78,000 miles,it's an automatic, 5.3 liter and 4wd (obviously). It's never been in the mud, or abused. I've read around on here a bit and evidently it's got the G80 axle code because I remember a few times spinning 1 tire in the rain and then BOOM the axle locked up and both tires started spinning. So by the feel of it, it's definetly a locker, not an LSD.

The other day it snowed a little and he had it in 4wd. He told me it felt funny a couple of days later so I went out, put it in 4wd and drove it (on dry pavement now). Pulled out of the driveway and instantlyfeel the strain on the truck.The inside back tire was chirping around the turn so I stopped mid-turn and put it in 2wd. As soon as it went into 2wd I heard a loud clunk, the rear end of the truck lowered an inch or two and the differential was no longer locked. So in 2wd, the differential if fine, but when you put it in 4wd high or low the rearend locks up. It's never done this before. I remember a couple of times pulling up a grass hill in 4wd then onto dry asphault and making a sharp turn just fine.

He dropped it off at the dealer 2 days ago. They test drove it and said something was definetly wrong so he left it there another day. Today they called and said they couldn't find anything wrong. I don't know squat about this differential but to me it sounds likethe differential itself isok because in 2wd it works properly. Is there some kind of accuator or something that locks the differential that could have gone bad or something? Any help appreciated.

Jason

P.S. The truck has never had any kind of drivetrain work before so it's all bone stock.