On our 1965 Mustang Project I have a clunk at low speeds. It isn't there consitantly. It appears to be only after the rear differential fluid has warmed up, and NOT there during straight ahead driving. The clunk is nearly the same as if the rear end had a spool. The fluid levels are where they need to be but have not been changed in quite some time. I plan to drain the fluid and replace with correct fluid.

BTW it is an original limited slip differential

 

Maybe the "friction modifier" has not been added to the oil? That will cause limited slip issues.

I would change the oil add the modifier, and made sure the YOKE is tight before jumping to conclusions or digging in any further.

 

after doing some figure 8's last night and putting it up on jack stands the yoke is tight, the u-joints are tight, and not much back lash between the driveline and wheels. the figure 8's revealed that the only time the clunking "kicking" were at low speeds with the front wheels turned and brakes applied. It would not reproduce straight with brakes on or off, nor at slow speeds with brakes off. The wheels come off this weekend to check brakes to see if they are adjusted right or hanging up. It definately needs to be bled as the brakes are quite soft.

I do however plan to drain the differential. I have new Amsoil for limited slip to add. I will update as soon as I get all of this done.

 

i'm with JMD. a trac loc clunking in corners is asking for friction modifier.

 

could also be the spline shafts... Had the clunk on my 2000 f-150 and all it took was some lube on the splines... cheap and easy to do. On the other hand, I had to replace the Limited slip on that same truck because the clutch packs went bad... felt like a detroit locker kicking in when turning.