97 Astro Vibration from Rear

My 97 had a howl in the rear when I bought it with 52,000. I just replaced all the bearings and seals in the rear except for the axle bearings. But the pinion and differential are all new. The howl is gone but I get a vibration at about 45 to 50 MPH and up to about 70MPH. I also had aftermarket wheels put on, so I had them rebalanced, New u-joints, new shocks, new brake drums & rotors. I still have vibration. I even pulled the differantial back out to check the bearings and they were fine. (Timkens) I did not pull apart the pinion though. We set the pinion with 15 Ft Lbs of preload. Is this Right? My problem has to be pinion or possibly the Tailshaft bearing in the tranny.
...Does anyone have any suggestions? The van has 94,000..if that means anything.