Bernie,
I have had Posi's in several vehicles.
Sometimes the clutches get to where they don't want to release and slip in turns.
There is a special lubricant that you are supposed to use in Posi rear ends.
If your rear end oil has been changed it very possibly is now the wrong type oil.
If we assume it is the right gear oil and it hasn't been changed in a while... I would recommend changing it and making sure it was the correct oil.
Then I would go by the dealer and pick up a can of the "SUPPLEMENT" that can be added to help eliminate the problem.
I know this, (but I appreciate you bringing it up) and that's why I was trying to clarify if it's a posi or not. That's my starting point here. AFAIK the diff has never been opened, and probably never had the fluid changed, much less checked. BUT! The real problem is determining if in fact it's a posi or not. 1999 was the year that GM started changing the 14 bolt rear to posi, using the Eaton design. However, this truck is something of an odd duck, in that what GM seems to have done is use up the last of the old parts on these cab & chassis units. Hence, it's old body style and the old headlight style (two sealed beams vs. the quad setup on the same year C3500's) and many parts for the regular C3500's from that year simply don't apply or fit on this; you have to go my the 1998 model year for many things. To make it worse,
I don't have the build sheet for the vehicle, so I can't look on that and see if it's posi or not. I do have the RPO codes, but all they tell me is the ratio (4.10). Also, from what I've read elsewhere, the posi units they made that year didn't use the additive, and adding it can do more harm than good.
Now, I
do believe it
is a posi unit, based solely on how it performed in the snow last year, and the twin ruts it dug in my gravel driveway trying to get up the slope when it was solid ice. But that's not what I'd consider "proof", just a hunch.
I suppose what I'm going to have to do is get the codes stamped on the axle tube, and see if I can cross reference them to identify it positively, or pull the cover and actually look inside. I'd rather not pull the cover, first because of the slope my driveway has, second, because it's my only vehicle right now and if I have any problems with it whatsoever (wrong fluid, wrong gasket, defective gasket, broken stud, whatever) then I have no way to go for parts. At least until the wife returns home from school Saturday.
Six of one, half a dozen of the other.
Edit to add: The reason the slope is a factor is A), because it will make it impossible to get the fluid level right when I refill it, and, B) if anything is broken inside and it has to be fixed where it sits, I don't trust the parking brake alone to hold it on that hill, even with chocks on all 4 corners. Fortunately, it's a full floating axle, so the wheels can stay on while the diff is apart, if it comes to that.
