Thanks all for the replies
The problem really became noticeable when we changed rear ends from a 7.5" with a 3.42 gear set to a 8.5 with a 3.90 gear set. Before the swap the 8.5 was completely rebuilt all new bearings, ring and pinion,johnny joint upper bushings,backing plates,brakes and hardware,drums. Only the housing,carrier, yoke and axles were reused.
The drive shaft from the donor G.N. was heavily pitted in spots so a Strange heavy duty unit was purchased.
Driving revealed a vibration so original pitted drive shaft was installed and the vibration seemed to go away.
Further testing revealed a very loose fit between the output shaft and the new slip yoke( this was confirmed by chucking a spare output shaft in a lathe and sliding the drive shaft on and measuring with a dial indicator against the original one,the new one had 3-4 times the run out, Strange then sent a replacement slip yoke with different spline geometry this reduced run out to less than the original) there was still a little vibration but it was known that the car dog tracked a little and the pinion angle was out of parallel by 2 deg.
The first of the week adjustable upper and lower control arms were added, thrust angle was corrected and pinion angle was set to -2.5 deg. which I believed to be the wrong direction as transmission is sitting 3 deg down.
Car was driven and vibration was the same or worse we then lifted the rear of the car with a floor jack spun it up at 65 mph the vibration started rear tires were removed this made no difference brake drums were removed and this made no difference. Pinion angle was reset to +2.7 deg. car was driven again with no noticeable change.
At this point all we know is
It is drive line related
It's not the tires
It's not the gear set as the 3.90 was swapped for a 4.11
It's not the brake drums
It remained unchanged with 2 different drive shafts
What we don't know is
Was it present with the 7.5" axle with 3.42 (it may have been there but at a higher speed)
And Where it is coming from all the easy stuff has been tried
I really appreciate all the help
Thanks Mike
