I have one of these on the 77 2+2 I am currently restoring. Apparently they were installed at the factory on later cars to address a "clunk" and/or vibration coming from the diff/rear end. They also fitted them after sales if the customer complained of a clunk or vibration end the rear. They are referred to as a diff counterbalance.
Don't know how effective they are, but from what I have read it seems the main issue was with the automatics, but mine (and a number of the other contributors to this information) is and has always been a manual. So mine probably added as a result of a complaint after sales. Hope that clears it up.
Cheers Paul