The car starts vibrating from 3rd through 5th gear and after about 40mph,
I have somewhat recently had the u-joints in the drive shaft retrofitted with replaceable ones.
The problem started as a weird clunking and rattling noise and then the noise stopped and the car started vibrating at speed. It's the whole car vibrating, not through the steering and it feels like it is coming from the rear of the car.
The vibration is only under power and completely stops in neutral, during shifting, and when the throttle is released.
I searched and couldn't find anything that sounded close..

Any ideas as to what's broken?

 

take the driveline out and check over it. Make sure the U joints are ok. COuld be worn CV Joints. gotta start somewhere.

 

Pull the driveshaft, check the U-Joints.

 

if the driveshaft checks out ok, its likely to be the diff.

 

Sounds like the driveshaft to me. You have the same symptoms I had.

 

You sure its not a wheel bearing? Does it sounds like you are driving on a rumble strip? Or will the sound change if you turn right or left?

 

It does make a vibratory kind of shuddering sound and does not change when turning but does only vibrate when I'm on the throttle like I said. I noticed driving earlier it also makes kind of a metallic clunk when I am on the throttle in low gear.

 

Check-out all your mounts - diff, tranny, and engine.

 

The metallic clunk sounds also was one of my symptoms before the heavy vibrations came. Get ur driveshaft checked out asap dude. If that thing falls out while your driving its not going to end well.

 

Driveshaft + front differential mount should get you back to normal. But rear differential mounts and other bushings should be saved for

 

We pulled the driveshaft and it turns out the people who replaced the ujoints before did not put grease in the fittings of the ujoints when they installed them and the rear one was also installed incorrectly so it was almost falling apart. Now i am stuck at family's place after thanksgiving desperately looking for a u joint and i will probably miss significant work :/

 

If you go back to the staked in replacements be cautious as they have a high failure rate. My front one broke two days after replacement at ~6k. Luckily the exhaust caught it before it hit the street.