Could very well be the differential mounts/bushings-get them checked before you pop for the bearings. The diff bushings made the same sort of noise, but only when applying the throttle; ease off the throttle, pop it in neutral & see if noise goes away.
In my experience, when these bearings go, they get real loud real quick. Good luck Neil!
PS I doubt there are HD bearings-the ones in there are huge!
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car