Subaru - Outback :: 2006 - Wobble / Shudder While Accelerating

Took my 06 Outback (106K miles) in to have the right front inner & outer CV boots/axle assembly replaced (one of the boots had torn open and was slinging grease everywhere, so no surprise there). When I was driving home after picking the car up, I noticed what felt like a wobble or shudder that seemed to come from the center or right side of the car (I couldn't tell which). It would only happen while accelerating, kicking in at about 20mph and then seeming to go away at about 30mph. It didn't happen at all while coasting, braking, or decelerating. The more of a load there was on the engine, the worse the wobble/shudder was, and if I accelerated very very slowly it wouldn't happen at all or would be just barely perceptable. I didn't feel it in the steering wheel; it was more like something felt through the seat and the car itself, if that makes sense.

So I took the car back to the shop the next day, and although the owner of the shop said he drove the car and didn't notice anything, he decided to go ahead and do the job over under warranty just in case they had installed a bad axle or had otherwise "botched" (my word, not his) the job. He also rotated and rebalanced the tires on the off chance that's where the issue was. Within about 15-20 minutes of picking up the car the 2nd time, I started noticing the wobble/shudder again, always under the same conditions as I've already noted--only while accelerating, always kicking in at about 20mph and going away at about 30mph, and always worse the more of a load there was on the engine/the faster the acceleration. The only additional info I've been able to glean is that it doesn't ever seem to happen when I drive the car first thing in the morning, but starts to happen as the car warms up.

At a complete loss, I crawled under the car and starting looking and poking around for something that might be loose or knocking around. I was particularly interested in determining if there was any play in the U-joints or some kind of damage to the drive shaft. I didn't notice anything except for this--at different points on the drive shaft there are what I assume are balancing weights welded to the shaft. Well, I found a bare spot toward the rear end of the shaft that seems as if a weight used to be there (although I can't say with 100% certainty). The drive shaft is painted entirely black, as are the weights, except for this one shiny, bare-metal, square-shaped spot. "AHA" I thought to myself...balancing weight broke off (although how that would happen beats the heck out of me since it's fairly well-shielded from road debris...maybe it was just a cr@ppy weld), now the shaft is out of balance, hence the wobble/shudder. However, the more I thought about it, the more I wondered...if the shaft was out of balance, why would it only wobble while accelerating? I understand why it might only do it at certain speeds, but if a shaft is out of balance, shouldn't it wobble/shudder at those particular speeds regardless of whether I'm accelerating, decelerating, or coasting? And why would it not seem to happen until after the car had been driven for 15 minutes or so and had warmed up? Again, it seems like an out-of-balance drive shaft is going to be apparent regardless of the operating temperature of the car.

So, could I be on the right track? And if so, is the fix as simple as welding a new weight on there? Can this be done with the shaft still on the car? I think Subaru considers the shaft a non-serviceable part, but I don't want to trash and replace an otherwise perfectly good shaft for what seems like a pretty easy fix (albeit one that is probably beyond my own abilities).