Ian, and others, Funny vibration

Truck is a 2000 S-10 ZR2.
I've had this for a little while now and it recently started getting worse. It's somewhat intermittant, and now it seems to happen at all
speeds, but it used to be specific speeds.
I recently rotated the tires and that didn't change it at all, it still vibrates and it still doesn't vibrate the steering wheel (tells me the front-end is good and the tires are not the problem, ball-joints are only 5 months old). I just did a pry-test on the rear u-joints, no play there, I used the biggest screwdriver we have, which is pretty big, maybe as big as a small pry-bar. I didn't pull the rear discs off to check for a dragging e-brake pad, maybe I need to, though I use the e-brake all the time.
Is it possible my DS is out of balance? I didn't notice any missing weights on it, but ya never know. This truck has a one-piece aluminum rear driveshaft, so it's not the center support bearing. Now that I think back, this seemed to have started shortly after the ball-joint work, which was done at my local dealer, they're close by, closer than any mom-n-pop shop I actually trust. I need to check the rear axle fluid, that may be tomorrow. Is it possible the CV's on the front axle would cause this and not shake the steering wheel?
I know I have some torsion-bar support links that are worn, the ones on the cross-bar the bars are tensioned on, but that's another clunk all-together.
I plan to be towing this summer, would like to get this resolved before then. Need to install a tranny cooler before then also, and get the tranny serviced.
TIA for any help, I know you guys will probably come through with the answer as I've seen it happen before, and helped make it happen a few times myself.
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Mike Levy wrote:

From your description, I'd pull the rear driveshaft right out and manually check the u-joints....you may have one starting to seize up. Which is hard to detect without actually removing the driveshaft.
Ian
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wrote:

I was thinking that as well, wasn't sure if it'd cause this type of vibration. I also get a clunk when accelerating from a coast or a turn, is that also an indication of this? Sorry, I forgot to put that clunk in my original post.
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Mike Levy wrote:

You may have a slip yoke issue (for the clunk). That's easy to check or fix when you pull out the driveshaft. Just lube the slip yoke splines when you re-install and see there is any change or improvement in the clunk.
Ian
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