'85 Yota pickup SR5 Short Box

At random times the entire truck will start to shake violently. I haven't been able to pinpoint a speed it happens at because it seems to be totally random, but mostly upwards of 45mph.
The steering wheel doesn't shake at all and I have no trouble controlling it, but the entire back end of the truck wobbles like no other for about 10 seconds and then it just stops.
Sometimes I can drive for an hour straight without it happening, and other times it happens right away.
My dad says the bearings might be shot and the ball joints are visibly leaking when I lock the front hubs in and drive for a bit.
It's really starting to piss me off. Is it difficult to repack the bearings? Or should I just take it in to have it done for a way high price

 

could be bent wheel or way out of balance

 

Jack up the rear end and spin the tires looking for any runout. Could be an out of round tire, bent wheel or a bent axle shaft or two. My '85 does this and it is bent rear axle shaft. As long as the two bends are out of phase with each other, ride is pretty smooth. But get them lined up, like going around a turn, and it shakes pretty good, you can see the passenger seat headrest vibrate. Go around another turn and the vibes go away. Rear bearings are sealed and need to be replaced if worn out, fronts can be cleaned and repacked.

 

Tighten all suspension components. I had the same problem after I installed leaf springs on my runner. Turned out the leaf spring bolts were hand tight. Problem went away after I tightend them up.