I really don't know, and no longer have a factory set-up to check.

Re-reading this post and others you've written on this vibration keeps bringing me back the driveshaft. Is it possible to borrow a known good shaft to try?

One other thing, when you checked the slip yoke for excessive play did you check it at the same depth as it sits with the d/s in at ride height? Its possible the tailshaft bushing is worn in one section as opposed to its entire depth.

 

I AGREE!!!! This feels so much like driveshaft I can't stand it!!! With the jackstand vib test seemingly pin pointing the vibraton to, output shaft, driveshaft, pinion yoke or bearings. The tailshaft/bushing sure seems tight to me (I checked it at ride height, with just a yoke and with the whole shaft) The pinion bearings are new and make no noise. Tried different pinion yokes with no change.... What else could it be? The rubber trans mount "softened" the vibration so I think that points me back to the driveshaft...
Unfortunately I don't have anyone I know right off to swap in a "known good" shaft. I'm working on it.
But, I'm going to call Denny's Driveshaft Tues and ship the LS1 shaft out to them. If nothing else, they should be able to spin it up to at least 6000rpm an tell me whats up.

If I'm at all lucky, I'll get it back together just in time to put it up for the winter!!!!!

 

your not supposed to tighten the pionion nut with an impact gun , infact it only needs about 25 in -lbs of preload . maybe the bearings in the rear are going . i had a very similar problem and i got a master rebuild kit for the rear and repalced all the bearings . my pionion bearings didnt look bad but were failing

 

I don't know what else it could be......
I ordered a new driveshaft from Denny's.... It will be here in three weeks. Then we'll see what that does.

 

Raising the dead here to see if this was resolved and how?

 

I have the exact same vibration, which I also cannot figure out. High speed only, above 75-80 mph. Gets worse with more speed and is seemingly fine under tthat speed. Did you find what the cause was??