Coke Man

I have a 1974 260Z that I converted to a V8 I also put a R200 rear end in it. I am now hearing a noise in the rear. Seems when it is pulling I do not hear a noise however, when I let up on it I hear a grinding noise of some kind. Could this be in the u-joints.

I am listed as Coke Man and I am new to the site. I have a 1974 260Z and the strips on the front and rear bumpers need to be replaced along with the rubber that fits up against the car behind the bumpers. I could also use some bumpers that are in great shape as mine are a little dull. Can anyone help. Also does anyone know where I can get some radio knobs for thr radio mine are missing. One other thing I have a R200 rear end and when going down the road as long as it is pulling it is quite however, when I let off the gas I rear a noise. Could this be a bad u-joint? By the way I really enjoy this site.