A buddy of mine has a 1999 1500 Silverado, ext. cab short box. We put some 285's on it and he got cocky wherever he went and climbed some hills and what not. Well the one night he got stuck on a road climbing and extremely steep hill and bounced the truck on the back end and I witnessed the frame flex far beyond what it should have. A week or so later he did notice that right above the rear axle he cracked the frame...that's fixed now though with two 1/4" steel plates inside and outside of the frame rails. This is not the problem though, I'm just assuming it could have caused it.

His rear end, yes...I know, "clunks" when driving. The truck had been sitting at a mechanis shop for over a week before I picked it up for him. Immediatly as I drove off it was terrible, constantly "clunking" and you can feel it in the truck, and it's definately the rear end. Once it seemed to get warmed up though it went away a bit, but when letting of the gas driving down the highway you could feel it slip again and the speedo kind of jerked along with the truck. It does it more so when turning. Now the codes in the glovebox, second row...the two columns farthest to the right read GT4 and G80. Now the last on means he has the locker, and I'm guessing this is what is causing the clunk. I think that basically his locker is shot and is going to eat the rear if he drives it anymore. Now is the GT4 a 3.73 rear end?

Please guys, if ya have any knowledge about this shine some info on it for me. He's being told to get a whole new rear axle and I'd like to find out if it can be a simpler fix yet. If needed I can pop the cover off and take a look for myself, I've changed axles and what not before but I've never fully torn into a rear end, bearing whise and all that.

Thank you guys in advance for the help.

I'll bet spinning and the bouncing he broke the spider gears or atleast got a tooth.

pull the cover off and see what falls out, you're not going to hurt it any worse than it is. check all the gears for missing teeth.

I sure wouldn't be driving it, loose parts get into the ring gear if it hasn't already may cause it to lock up. could wreck it or worse.

Yes GT4 = 3.73 gears. I wouldn't drive it until it is fixed but unless the axle tubes/center section is physically damaged I can't think of any reason to buy an entire replacement rear end.