According to my manual, the locking rear diff will ONLY lock when in 4LO, and it will automatically unlock if speeds are over 20mph, or if the vehicle is shifted out of 4LO. It also states that it will automatically unlock the rear diff if the battery is low (it's an electically locking differential) suggesting that the "fail default" would be unlocked. The fact that the wheels turn opposite directions when up on the rack tells me that the rear diff is NOT locked.

I think the issue is with your center diff. If that is locked then it will grab and shudder in tight turns on the pavement. 4HI LOCK should only be used off road.

 

BTW, the difference between 4HI and 4HI LOCK is the "center diff", which is part of the transfer case. 4HI LOCK has nothing to do with locking the rear (or front) diffs, but rather the center diff. 4LOW LOCK also locks the center diff (but not the front/rear diffs) but also uses the lower transfer case gears.

 

The symptoms you describe in your first post almost sound to me like the normal actions of a vehicle in 4WD. Tight cornering in 4wd on pavement under throttle will cause wheel grabbing/chattering/shuddering. If you are experiencing these symptoms in 4HI LOCK but not 4HI, that is normal. If you are still experiencing them under 4HI (not LOCK), then it points to the center diff not unlocking.

In the case of the H3, 4HI is similar to AWD rather than 4WD, with a center diff. 4HI LOCK locks that center diff and it acts like any normal 4WD truck. 4LO LOCK also locks that center diff as well as giving you the lower transfer case gearing. Finally, locking the rear axle locks that rear diff.

20" rims shouldn't have any effect (the tire diameter is still going to be similar to stock), the rear diff could not have been locked in 4HI, and would have unlocked once over 20mph anyway.

 

Thanks for the replies and great info 650Hawk, much appreciated.

Since starting the thread a bit of work has taken place. We decided to get into the rear diff after all and it was locked. The magnetic locking mechanism was locked up due to a c-clip retainer which had popped out. (inside diff. housing, main axel bearing, outside face...hope I you can envision that). It didn't solve our problem but at least it wasn't a waste of a job.

We then went to the transfer case, found the chain pretty loose/ a lot of play, checking it through the drain plug w/ camera and finger. Pulled out the transfer case, opened it up and found that a ceramic(?) gear selector ring had broken. Half of it was at the bottom of the case while the rest was pulverized and all throughout the bearings, gears, oil ...bloody everywhere. We just finished ordering the parts for a rebuild to put it all back together again.

 

#16 in the diagram was busted, first piece, left, in the grouping.

 

Wow, so you had problems in both the rear end AND the transfer case. Bummer. Well, at least you'll know that the innards of those two components are good after this!

 

Yup, a bit of a mess. Brother-in-law rents out his cars and the Hummer is one of them. Someone decided to do some off roading, it seems, and from what we could guess (circumstantial evidence) they got stuck, gunned it, it grabbed and bang....can't really see another way of doing this sort of damage.