OK, I experienced some driveline vibes, and put in a 4 degree shim this weekend. Edited the post above with notes about measuring driveline flange length and the info below:
EDIT: diagnosing driveline vibration. Now, I had some driveline vibes in the low teens, from 25-30mph, from 55-60mph. I measured the driveline flange distance and got 51&5/8" TOP and 51&1/2" BOTTOM - so the top is 1/8" longer than the bottom - DIFFERENCE of +1/8" top to bottom. We want to get back to the original stock measurement DIFFERENCE (you measured it before, right ;-) or back to a difference of 0" or maybe -1/16" (bottom 1/16" longer than top)
I put in a 4 degree shim and got new measurements of 52&1/4" TOP and 52&7/16" BOTTOM for a total difference of -3/16" -- TOO MUCH negative difference. No surprise that on test driving, I've still got driveline vibes - at a slightly different sound and slightly different speed. Remember, we're only interested in top to bottom differences, but note how that little 4 degree shim made a 3/4" total difference in driveline length!!
I'll order a 2 degree shim to split the difference, and swap that one in for the 4 degree shim next weekend that should get me back to zero. or -1/16" Stay tuned.