on the rear of the differental there is a bar that resembles a "handle bar mustache" It mount to the diff by two bolts in the center of the bar and the outer ends mount to the rear frame rails. The bushing that mount to the rear frame rails are the ones to replace.
I replaced mine about two years ago. What a difference. The car handles much better off the line. This would also be a good time to drop the rear end and open the "hog's head" to check for wear, as well as replace the oil seals.
It also follows that if you have dropped the rear end, the drive shaft will be out, great time to check the u-joints. (make sure you mark the drive shaft and output shaft positions before removal)
Yes most of my $20 projects turn into $200 projects quickly.