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

Hi ang94541
I am glad I am not the only one sitting with this problem. My car doesn't see the freeway that often as it is only driven over weekends. This is the reason why I can't justify the price it is going to cost to get a 5 speed box or even repair this problem on the 4 speed. We have a freeway speed of 120km/h here in South Africa but I can take it max 110km/h and then I can feel the vibration starting to come in.

Another guy here at work told me exactly the same idea you have. Making a tool whereby you can get the old bush out and then pressing in the new one. He also suggested to use a flat piece of wood when hammering in the new bush not to damage it. The one gearbox shop I phoned recommended that I do not follow this procedure as it can damage the bush. Hell, the price they are asking to pull the gearbox, fix the problem, new gearbox oil and put it back in I am definitely not going to use them.Especially with the possibility of damaging my GX manifold when removing the exhaust from the manifold to get space for gearbox removal.

If you really struggle with the bush on your side, I can get one for you here brand new and we can mail it by sea to bring postage down to a very low price. Let me know.

Yea, 110km/hr is about 68mph, that sounds about the same as mine, in 4th and 5th gears. I found that after I have the driveshaft fitted with new u-joints/straightend/balanced, most of the vibration goes away. It still leaks though.
The thing is, the output shaft is in the way. If something goes wrong or the yolk doesn't fit you can't hone it. You can't really measure the ID either. So you're screwed and the whole thing has to come out.
I emailed NissanParts.cc to see if the bushing is still available. I can PM you about buying one for me if need be. Thanks.

Also, Peter's bushings don't seem to be showing up anywhere either. Darn.

See what you can do on your side and if you struggle, I am willing to help. I have mailed a couple of stuff so far and by sea is quite cheap.Takes 3 months but for the money I think anyone is willing to wait a bit.