I flipped the driveshaft 180 and it might have improved things... The vibration doesn't start until 40mph instead of 35, and may be less violent. Just run it for a month and see if anything changes?
I guess I'm not visualizing what you are describing. I've got the volvo greenbooks on CD, I'll try to find what you are describing.
OP, did you properly clock the drive shaft plate at the rear of your trans with the plate (and U joint) at the rear end flange? The green books show how this must be done. Basically they must be opposite......if the rear pin on the plate is horizontal, then the front must be vertical.
This clocking is separate to the center joint where you disassembled the halves..... I’ve seen some factory match marked, some not, and late versions are machined so they cannot be improperly assembled.
I guess I'm not visualizing what you are describing. I've got the volvo greenbooks on CD, I'll try to find what you are describing.