knock in differential

first off, an introduction is always nice. car mods, pics, etc.

second, welcome to the boards.

third, when was the last time you check the oil in there? you ever had anyone in there doing anything? gears?
 
Yep, if you are sure it's the rear end, nothing you can do but remove the cover and see what's going on. Nothing you can see from the outside (unless your driveshaft has fallen off, or the pinion nut).

Once you get the cover & DS off, clean out the housing, and check for anything obvious.

Check to see if the differential shaft locking bolt is still intact, and the differential shaft is still intact as well.

Then get some gear marking grease, and check the meshing of the gears.
Scott
 
Can be many different things
Big backlash in spider geard
Differential clutches worn out and you got little biger backlash there
Pinion or gear worn out
Rear or front U joints
Or gear in tranny worn out

I had same problem but It fixed with Eaton diffearential and Superior 31 spline axles