Clunky C-3 Differential ???   When I come to a stop and then make a slight turn, my '75 C-3 CLUNKS like something in the differential is out of sync....not engaging properly.  It makes a heavy clunking noise that goes away when driving straight. (I think its a posi-traction rear end.)

Any suggestions on what the problem is and how to fix it??  What do you think repair might cost at a good garage??  

Well, a remanufactured differential can run around $1000, probably similar or less to rebuild your current diff.  However, I suspect it's possibly just a simple U-joint.  I just had the stock suspension completely redone on my '74 in May, and 3 of the 4 U-joints were completely worn, one of which was actually broken.  But the differential was in perfect order.  But basically the simplest & smartest thing you can do right now is just take it into a shop and have them look at it.  

I have the same thing and my U joints were fine. It's been suggested that I replace the fluid in the diff, and add one that includes an anti stick clutch lubricant, as the clunck my be the positraction clutch sticking. Worth a try for 60 bucks rather than a new diff  

Any GM factory posi unit has a certain clunk to it, but if the proper additive isn't in the rear end lube, it'll be worse. I'd definitely recommend a fluid change for the rear, with the proper bottle of posi traction additive included.  

"Any GM factory posi unit has a certain clunk to it, but if the proper additive isn't in the rear end lube, it'll be worse. I'd definitely recommend a fluid change for the rear, with the proper bottle of posi traction additive included"

Ain't That The Truth.  Beats buying and having a clutch pack and new springs installed and then paying twice that amount to the guys who turn the screws for labor.

Lucas makes an additive I used in my 66 Goat which had a 67 Buick GSX rear end in it.  It to was positraction.  I am not sure though if it passes the GM approval ratings though.

RedCorvette86  

Sounds like a good idea.  Mine does it too.  I got under it and looked at the u-joints and they look fine but im not a mechanic either.  I just got it this summer and replacing that fluid may be a good place to start.  Mine does the EXACT same thing.  Let me know what it turns out to be if you get it checked before me.