Whine, is it in 8 inch?

New here so bear with me. 66 Fastback is 98% complete and just got the new 2.5" Magnaflows on. So I've started driving her more on the highway and the whine is getting worse, you can definately hear it over the exhaust. Seems to be the loudest around 2000-2200 rpm, but you can hear it through most of the rpm range. If I let off the throttle, the whine seems to decrease.
It's an 8 inch rearend with 3.50 gears? ( determined by using chalk mark on tires and drive line and counting revolutions ). Freshly rebuilt C4 with B&M shift kit and 1800-2000 stall.
I don't think I'm mechanical enough to try fix this myself, just looking for opinions as to what the problem might possibly be. It would be really neat if you gave me an easy "Duh, heres your sign type fix." Thanks Brad.

The whine definately seems to decrease if I let off the throttle. It might decrease a little if I accelerate. I didn't think to take notice during turns. I have not yet tried coasting by putting it into neutral when it is whining the loudest (which by hte way is loudest at 2000-2200 in third gear, slightly less loud in second, and faint in first. Probably shouldn't admit to this, but at 5000 rpm and 95 mph I don't hear much of a whine). Here is some more info that may or may not help: since I bought the car 5 years ago I've done nothing to the rearend except a complete brake job, new seals, drain the case and refill. Prior to a couple weeks ago I had not driven the car more than two or three blocks at a time, just from storage to the body shop and back. Thanks, brad .