trans/diff problem

Transmission or differential problem on a 1972 240z with 160000 miles on it, car has sat for 8 years, bought it 4 months ago, got it running, doing about 40 miles per hour and grinding noise starting coming from the rear passenger side differential area. Noise only when I try to excellerate but all the gears seem to work and shift ok. Before I start to get into it, just wanted to get an idea what to look for or what the problem could be. Have not added or changed any fluids in trans or differential. I am wanting to fix as much as I can myself and keep original.