Looking for some help diagnosing a major rear end shudder/ vibration between 35-45 mph. Doesn't matter whether I'm on the throttle or not. It's a 99 4WD V6. 140k miles.

Things I've crossed off the list:

Unbalanced wheels - swapped with a known good set. No change.
Drive shaft - pulled rear driveshaft and test drove with FWD. No change.
Replaced rack bushings. There was major play there and steering was very loose. Steering is now tight but no change in vibrations.

I noticed applying the E brake made the vibrations worse while at that speed. I've visually inspected and didn't see any broken linkages or completely toast bushings. I've also checked the rear diff for excessive slop, seems within spec. I'm thinking warped drums, shot bushings, or something wrong in the rear axle. Really hoping it's not that last one. Anyone have experience with something similar or an idea to point me in the right direction?