I fairly certain that I have the exact same grinding sound. It happens when the engine is between 2000 and 2500 RPM in 3rd or 4th gear, clutch in or out, basically when I'm coasting at about 45-50MPH. It started happening after I installed a NOS 3.91 differential! I've replaced the wheel bearings, all diff mounts and bushings, including the mustache bar bushings. I also have the dreaded rear end clunk happening when quickly letting out the clutch. Could it be something rattling in the half shafts? I also suspect a rattling exhast pipe, but I'm not sure....