Then I'd start at the rear wheels and spin them and trac ethe movement all the way to the trans. Put some resistsnce on the drive line as you spin - spin rear wheels and grab each of the wheel drive axles (with fingers or with needle nose pliers) and make sure they resist. The to the rear center drive shaft - spin wheels and put some friction on the rear center.

Prob should be between the trans and the rear wheels. I'm not aware of anything in the trans that will just affect the rear. If its in trans it would afect both F&R.