When cruising at 75mph, depressing the throttle causes a sort of vibration or droning. Low frequency, constant.

I went to 2WD and it still happens, so it is not the drive shaft, rear axles, or rear wheel bearings. I pulled the two front axles and sent them to a specialist to be checked - nothing wrong.

All mounts are okay - the rear tranny mount is new.

This is frustrating! What else could it be?