There are different vibrations..they tell me different things. A vibration felt in the seat or that puts your feet to sleep is driveline..or something spinning fast. If the steering wheel shakes it is usually a tire balance problem. I know I am just a Jr. member,but I am a retired ASE Master Tecnician. I made my living doing Alignments and tire balance. Thats why I suggested jacking up the rear and running up to whatever speed it vibrates at. If it does that eliminates the front end. next,remove the rear tires. If it still vibrates remove the brake drums. If it still vibrates,you can about bet on the driveshaft or a clutch dish that has came apart.
Anyway, before you start buying parts, find the problem.