Toyota truck driveline vibration?

It all makes sense alright but it is a bunch of bull. Your mechanic should have removed the drive shaft and checked the pinion nut for tightness. He should have also checked the fluid level because if the pinion shaft was that loose to move up and down it would ruin the seal and it should be leaking or maybe it is so low it is not throwing the oil up onto the bearing as it should. As for the bearing balancing it's self that there is where the bull is. The vibration is got to do with the load harmonics. You should be able to get the same reaction on the same hill at the same speed and load. At any rate if there is play in the shaft you need to have it repaired. The bearing in question is very easy to remove for inspection and shimmed if necessary without tearing down the whole rear.

A loose pinion bearing does cause a sound on deceleration when it gets loose enough. It's kind of a growling sound that goes away on acceleration. The pinion bearing can sometimes be adjusted without replacement with the large pinion nut that's accessible after disconnecting the back end of the driveshaft.

Check the four bolt-nut combinations that attach the drive shaft at each end for tightness. I had a mysterious vibration at speed that ended up being loose nuts and bolts at one end where the drive shaft yoke connects to the pinion flange.

Inspect the drive shaft for any dents or major scrapes. I scraped a rock with the drive shaft once that re-shaped it enough to knock it out of balance/round. Replacing the entire drive shaft with a salvage yard piece took care of that problem.

Also inspect the U-joints at each end of the drive shaft. If they get sufficiently worn, they can introduce a wobble. Worn U-joints also often cause a "clunk" when going between forward and reverse.

in support (i agree),

not that these guys need my hep.

snippet,

food for thought, other possibilities you might be thinking of,

ring/pinion typicaly slap/clank. which on this, is part of the input yoke to the diff. especially noteable between loading and unloading torque apps. as stated, four bolts remove the drive line from the yoke.

in neutral, grab the yoke. turn it. hear any slap/clank? that would be ring/pinion issue. thing is, that generally doesn't put anything out of balance. so....

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a clue on a diff issue might be to check the rear gear oil. level/clean is the point. if anything in there is bad, other than the ring/pinion, should see some particulate matter in the gear oil.

2wd non extra cabs (short beds) don't have center carriers. so good there.

any prior related maint done on this? if the shaft was removed, and not match marked, therein may lie the issue.