when im about or above 55mph and i let off the gas to coast the car's rear will vibrate and shake a lot i just need to know what i probably need to replace or do to fix this.

 

Could be a lot of things:

1. Wheel out of balance
2. universal joints on the drive shaft worn
3. Loose or broken links on the rear axle.
4. Brake components broken
5. A transmission issue

You need to do more diagnostics to see what it is.

Does it happen in a gear ? What if you push in the clutch ?

If you put the brakes on does it stop or reduce at all ?

Can you do it with the car on jack stands ? Then you can eye ball the problem and maybe see whats jumping around. Make sure the jack stands are on the frame ( I usually put them under the mounting points for the lower links ) not the axle, so you don't hop off of them.

 

10 bucks says its the u-joints on the drive shaft!!!

 

Drive shaft's rear universal joint is my 1st thought.