I seem to get a howl at 70-75 on the highway. I have replaced the badly worn side gears and pin, the ring & pinion looked ok. This didn't get rid of my howl but did get rid of the nasty clunk going into reverse. Would anyone have any sugesstions as to what could be my culprit?

check your wheel bearings, mine howled when they were bad.

Thanks Mike, I'll do that. I have put new u-joints etc. in also , so that could be it.