I have a wobble in the rear of my voyager, and it gets worse after travelling a distance. What could be the problem?

If it wobbles with your hands at 9 and 3, its a tie rod end.
If it wobbles with your hands at 12 and 6, its a wheel bearing.

probaly a wheel bearing wearing out. becones worse when it gets hot

This is different. In a perfect world, you would run the truck on a dynamometer, get it up to 55 MPH and look under the truck. The fluid is going somewhere.
There is a vent tube on most axles. Follow the tube to find its' end and see if the fluid is traveling away from where you had expected a leak. Check your axle shafts under the rear rotors to see if the Emergency brake drum is oil coated.
Another way to think about this is the original failure. The pinion seal. The seal contacts the rotating flange. If you have a wobble in the driveshaft, (balance issues, U-joints) then the rotating flange can wobble. If you have a bad pinion bearing or carrier bearing, it can work to make the spline gear wobble.
So I am suggesting that the leaking Pinion seal was caused by something and the seal was a casualty of an undetermined cause.
You should be able to add an Ultra-violet dye to the axle fluid to find your leak. Even if it is running from the same place, the current leak will be showing the UV dye.
Please rate my info.

Rotate the tires and see if it follows. If it does then u need tires or u might have a bent rim. If it doesn't then check u joints, leaf springs, u bolts, bushings.

I do not see anything about the tires in your description of the problem. Might look into those things because they are what cause "speed related" issues when you drive.

If it works well at speed and acts up when slowing or stopped it doesn't sound like fuel filter or pump. You can try running injector cleaner through the engine to clean the carborator injectors or fuel injectors if you have those. Look at replacing the plugs and wires on the motor.

try doing a tuneup and do a fuel system treatment change fuel filter and airfilter if not already done. see if these help your problem on the nissan. on the voyager is sounds like it has a short in the motor pull rear panel off and is if a wire is grounding out causing the problem

dont park it in the sun or your tyres deform ?? sounds like they are deforming yet again .Common problem here in spain

check for a bent rim or axle. you will have to have on a rack to see.

If it seems to happen more when rounding a curve than on the straight, it may be your CV joint. I'm not sure but I think you can replace the half shaft that connects the wheel to the engine. Otherwise, you may have to find a press to press out the old one. It's kinda like a U-joint on rear wheel drive cars. Or it could be the tire out of balance, bad tire, bad stabalizer bar. Gonna have to jack it up and get under it and start shaking stuff to find out what's loose. GOod luck and have fun....