Rear End Noise

I have a 94 z28 with a pretty much stock rear end. I have a rubbing noise coming from the rear end. I thought it was brakes at first, It's a sound that is similar to pads rubbing against a warp rotor. I replaced the brakes all the way around with drilled and slot ported rotors. The sound is still there. I jacked the rear end in the air so both rear wheels were off the ground. While the car was in neutral with the wheels rolling, i didn't hear the sound, once I dropped it in drive i hear that rubbing sound again. Any idea what this could be? My last guess would be bearings but before I spend the $$$ I want to see what y'all think.