Elusive vibration starting 55 and continuing to highway speeds

[spyro235]

Car has 220k and likely that the tension struts need doing regardless....

The shimmy definitely gets a bit worse on lift, with the weight shift forwards I guess. It kind of comes and goes all the time, some drives i'll be pretty smooth and some drives it'll be a vibrating mess. When on heavy throttle, it does pretty reliably go away though. Turns don't seem to affect it whatsoever. Especially not making it worse.

There doesn't seem to be any weird wear on the front tires though, so it does come as a little odd to me. I need to do my front springs/ shocks anyway, because they're chattering recently over hard bumps. Maybe I'll pop new tension struts in there when I'm there anyway sometime this summer.

I'll get under the car before that, and give all 4 axles some good wiggling and see if anything moves at all. Maybe I'll Bend some heat shielding out of the way and check how the rear driveshaft and CSB looks. I can't see the joint with the heat shielding in the way. I've never seen a bad U joint but I'd imagine it would have a bit of noticeable play in it, right? The CSB would be obviously cracked/ shot rubber,

If the U joint on the driveshaft is bad, is a new driveshaft the fix? Or can they be rebuilt?