Almost immediately after I left the diff shop for the gear change, I felt the resonance on the highway above 60mph.
that's what Sherlock Holmes/Columbo and lieutenant Kenda would (all) call a major clue. Keep It simple Sam. Unless they messed up the shaft somehow during the swap (ujoint clips/caps can be delicate/problematic) I'd be thinking about swapping the pig at least for a test even with an open gearset. I know it is a pain.
Yes, I had thought of that, but, being that it is not an 8 3/4 with a removable third member and is a 8 1/4, that isn't exactly easy. The 3.21 setup previously had a Spicer limited slip added and functioned great. The 3.91 gears in it now were slightly used (about 2000 miles, but all teeth were in great shape). The same limited slip unit with 3000 miles on it was used. Very slight quiet whine from gears at any speed, but not louder on deceleration like what would occur with bad pinion crush sleeve.
What about an axle bearing being knackered a bit during the gear swap?
As mentioned, the problem really isn't a vibration more than it is a deep, almost cyclical hum that you can feel resonating mostly below the driver side seat and mid to front of drivetrain. Not to say its not felt further back as well.
