Rear vibrates when going >40mph

[kinfer]

Hi there Nico, well my problem right now is that today I got on the freeway and for some reason once hit 40mph, the car began to vibrate. The front brakes are fine, the rear need new brake pads and my right axle has been making a wierd noise for a long time. I have a welded diff and a manual transmission. I was wondering if the problem could be that the diff needs oil? The car doesn't shake when I press the brake so I don't think the brake pads could be causing this. What do you guys think the problem is?

[AZ89two4Tsx]

Are your tires out of balance?

[KDashy]

Bearings maybe.

[AZ89two4Tsx]

Try flushing the engine coolant.

[Nacho_240]

Try flushing the engine coolant.

wtf

First thing that came to my mind was a loose driveshaft or old tires.

[ViolentRunning13]

Balance.

[kinfer]

I was going to check the driveshaft today, but my friend is borrowing my jack. It's wierd because everything was fine yesterday. If driveshaft is fine, where else should I look? I took a peak of the underside from the side and the diff seems to be covered in oil. But then again my whole engine is took (leaking oil =P). I really doubt that coolant is the problem. I reved the engine a little bit after I pulled over to check it out, and motor seems fine. It's only happening when going >40mph. I'm broke right now...literally, so I can't spend any money until Friday. So I guess I'll check driveshaft, then I'll try to change the diff oil, then I'll get a balance. Is there anything else I should add to my to-do list this week?