When under load - the pinion may climb a bit due to spring deflections- and there are tapered shims around that will allow canting it down slightly (at rest) so that it climbs up near to the right position as the car is driven.

Jim, i have fitted a few of the 9" diffs into 56 fords and not sure whats causing the vibration but could be tailshaft and uni,s, more than likely. Normally the diff saddle mounts need to be replaced with the original style saddles to fit back onto the springs correctly, and moved about 1/2" inwards, (if i remember correctly)and rewelded, this is important step where about 5-6 degree pinion angle should be set,  I must add that i have seen the springs levered across to fit the 9" diff in! not good!  What did you use for tailshaft and does it run true when car is jacked up and running in gear. Will be interested in what you find to fix the vibration.

Jim.  Let me know if the tailhousing bushing is indeed the problem.  If it is, I’ve an assortment of tail housing bushing removers and I’ll see if there is one here specific for your transmission and if so, I’ll drop it in the mail.  I’m assuming you have the 28 spline tailshaft.  Be sure to check the driveshaft yoke for wear also.