You can also have someone watch the rear differential pinion while you start to drive away and they can tell you what it's doing.
At least it's one more bit of info you're getting.
Not to take any scrutiny off of one area, but you would most likely feel it in the rear even if it was the clutch. You're driving with that end anyway.
In 4-low you're not as likely to get any bouncing since, at least in first or second gears, you're hardly even required to use the clutch. It'll practically drive itself in low range and you're not putting as much torque into the twisting motions. Sure, you're multiplying the torque of the engine, but the drivetrain is getting it more effectively to the ground without anything in-between resisting as much.
When my clutch was going out from old age, I used to leave it in 4-low just to move the truck around. Hardly even used the clutch that way.
Hmm, but still, with that in mind, it still sounds like a driveline/clutch issue. The comment about the clutch linkage being between both the body and the frame was a very good one. But you're saying it only helped a little? Is the clutch adjusted properly now, as far as you can tell?
Anyway, have someone (or you) watch the rear end when you take off to see what's going on and let us know.
And while you're at it, leave the hood open and watch the engine while it's bucking to see if it's rocking in it's cradle too.
Paul