vibration under acceleration - flex disc?

[ncsucarjock]

Folks,

Was towing the trailer yesterday, and working it hard merging onto the freeway, doing a solid flat-out pull through 2nd and 3rd gear. 2nd seemed to be a little rough, but the top end of 3rd had a *lot* of vibration.

Later on, I disconnected the trailer, and took her for a spin. The same issues persisted, with 3rd and 4th, in the upper ranges, being shaky enough that you didn't want to stay in the throttle. Definitely in the driveline. No vibration at low speeds, or at highway cruising speeds. If I downshift and bring up driveline rpm though at cruise speeds (3rd at 70mph) it definitely re-appears, regardless of being in the gas or just cruising.

So something is going on... And the only thing that comes to mind is a flex disc. All new front suspension, and new 4 wheel alignment. Car is smooth otherwise. And it came on rather quickly, I don't recall any vibration in the past month, and I fairly regularly accelerate at those speeds.

I do know that at high speeds, and high rpms (above 100+) it's always felt a little "trembly". Maybe with that amount of force on the flex disc, it's been slowly degrading over time, and now has reached the point where even in "routine" driving, it's apparent.

Thoughts?
Evan