Rear Diff. locking up

Hi, I've got a 99 GMC Suburban, 2500, 4WD, Vortec 7400, 100k miles, towin pkg, etc. I'm 95% sure it has a limited slip diff. The rear end is locking up all the time. It's very noticable around a tight corner. I made sure the 4wd is disengaged (checked the engaging actuator in the front axle) to elominate that as a problem. I changed the fluid - used the correct (very expensive) GM gear oil. Did not see anything out of the ordinary (broken pieces, etc.) in the housing. I have tried making "S" turns in reverse, forwards, backwards/forwards, driving in circles, etc to try to free up whatever is stuck. It refuses to release. Anybody have this problems, any ideas, please? It does not make any whining noises or vibrations, the only noticeable problem is that it is not doing the differential thing around corners. Help - I'm stumped :confused:

Thanks for the help. Dito on the intake manifold problem. It's also a problem on the big block engine.

add 2 bottles of the GM limited slip additive.

I used the specified GM fluid ($26.00/quart) that is supposed to be for that rear end and have the additives. Are you saying I need to add 2 more bottles of additive to that fluid?

I think you used the wrong lube...it sounds like you installed synthetic lube.
here is the correct part number
Rear Axle (Locking Differential)
SAE 80W-90 Axle Lubricant; use only GM Part No. 1052271. Do not add friction modifier

Rear Axle (Locking Differential)
SAE 80W-90 Axle Lubricant; use only GM Part No. 1052271. Do not add friction modifier

why would you NOT use friction modifier on a limited slip rear?

I know some the GL5 lubes say not necessary, but then they chatter.

The part # they sold me is 89021677 - sounds like the wrong stuff @%$#^@$. Is that rear end pretty bullet proof, or are they prone to doing this. I figured new fluid should fix the problem. I do a moderate amount of towing, about 3-4 trips a year. I'll go back & get them to straighten this out. THanks.

why would you NOT use friction modifier on a limited slip rear?

Gm has determined it is not necessary with this lubercant


The part # they sold me is 89021677 - sounds like the wrong stuff @%$#^@$. Is that rear end pretty bullet proof, or are they prone to doing this. I figured new fluid should fix the problem. I do a moderate amount of towing, about 3-4 trips a year. I'll go back & get them to straighten this out. THanks.

Sounds like it.
Those differentials are fairly bullet proof.
When I service a differential, after draining the fluid (cover off) I clean the inside of the diff with some brake cleaner to remove any old dirt that may be still there.
After I fill the diff, I drive it a nd immediately do 3 or 4 turns (usually in a circle one way then the other) to work the lube into the clutch packs