a833 OD vibration in 4th gear at 3000 rpm

I just confirmed the vibration is still there in Neutral at 45mph, rpms dropped to 1000. I guess I'm going to take the new driveshaft back to Gregory's and have them check it. But, I suppose I could take the new tires and wheels off the rear , put it on jack stands and see if the vibration is gone. Hell might remove the drums too. Perhaps a brake drum is causing the vibration. I had to beat one drum off before I had the diff shortened and a piece of cooling fin broke off, probably 2inches long. Wouldn't that be great if it were the reason. Using Dr diff axles with green bearings btw. Using additive in the diff also.