I am having a heavy vibration from the rear(?) at around 75+ MPH... doesn't matter if im accelerating or decelerating. Clutch in or in gear.
I can feel it in the entire cabin (not in the wheel though).

I don't believe its the tires / rims. I just put on new tires, and it still does it (before and after).
Doesn't matter if the car has been running for 10 minutes or 30.

No audible sounds (whirring etc).

Any ideas? It gets pretty bad around 83mph.

prop shaft? warped rear axle(s)?

I recently purchased this car, so i don't have a complete maintenance history.

 

How low are you?

 

Good question. Now, only -1.5 below stock..

BUT, when i got this thing, it was pretty dumped (BC's set full low almost).

 

You may need half shaft spacers. Or just swap the cv buckets on the axles.

 

This is usually apparent only on acceleration.

Op what wheels did you put on? Centering rings used? A long time ago in broke college days I put a set of rota track r's on an Integra and had the same issue which the hub rings cured for me. Are you using the right lug nuts for your wheels?

 

OEM rims / Lugs.

 

I got the same issue.

 

do you have a hardtop?

 

I do not. I did find out the rear rim is bent. So that might be it. It balances, but still.