Yea, I have this probem too. I've changed the rear seal several times in the past and they all leak. I also have a vibration problem with the slightest wear on the u-joints. I found that if I grab the yolk, I can wiggle it around in the trans. I later found that the yolk was worn. I measured the diameter in three places along the length, but its worn enough such that you can feel it.
I swapped the yolk with a spare in good or atleast better condition and the fit is better but still has too much slop.

I've made a tool that I can use with a slide hammer to, IN THEORY, remove the bushing while the trans is still bolted together. The problem is I can't seem to find new bushing.
I believe the stock part is Nissan PN 32132-18000 BUSHING, rear extension.

I like Peter's fix, but I'll probably wait until I HAVE to pull the trans for something else and take care of the bushing then.

Here is Peter's write up...
Repair bushing for 56, 56A and 60 series tailhousings