Hello,

Kris, KDADDY792008-09-16 06:32:44

Did you put in the Positrac additive when you changed the gear lube? If not, you need to do this asap, before any damage to the posi clutches occurs.
I've never heard of a diff unit squeaking, but I guess it's possible. Usually, a squeaking noise from the rear is a u-joint, or t-arm/axle bearing. Then again, the posi unit will make noise while turning, usually after driving straight for a few miles. I'd suggest putting in the posi additive first, and go from there...

O.K. looks like some progress, Joel- I did not put any additive in. So my plan is to drain enough to  get the bottle of additive in, drive it until the weekend and drain all of the fluid and replace it with gm lube and more additive. My question is what is better? The Gm differential lube or a good quailty synthetic?

I had this exact same problem with my 79. I too initially suspected the differential...changed all the fluid...still had the problem.

Checked the U-joints...all but 1 was shot   I'd agree with Joel and have those half-shaft U-joints checked.

-Shane

The posi will normally make a grinding noise, or a "chatter" that you can feel as you turn. As I said, I've never heard of a diff making a squeak, but....I ain't the sharpest tool in da box.

A u-joint, if dry, will squeak at certain times, usually when the tires are rolling slowly, such as when you turn, or when first taking off from a stop. A wheel bearing will squeak the same way.

[QUOTE=Corvette77]After a normal run of about 20 mins or so when I make turns in either direction the rear end wines and squeaks. Once the car is put back into a straight line she is fine.[/QUOTE]

This description leads one to think it is the posi unit...perhaps the description of the noise itself needs clarifying?
As far as what brand of gear lube to use, any name brand will work just fine. Most of them will say that their lube already has the posi additives in it, but....I doubt that whatever they have in it is compatible with our older cars, and posi units.
The GM posi additive is really the best bet...two bottles is recommended. I also use the posi additive from Lubegard, and have not had any problems with it in all of the years I've used it. I couldn't say if other brands are good...I've never used anything but the GM, and Lubegard additives.
hth

One more thing to add.   Check the end play in the output shafts/yokes coming out of the diff.  If there is too much play the posi clutches will make terrible noise, worse when warm.   Just like mine.   Yes I have new ones to put in after the paint is complete.

Diff Saved!

Ya just gotta love it when a fix goes that easy!