Sources of vibration at high speed in and around rear wheel
Note: I've included things not at the rear of the bike as sometimes we think something is coming from one area when it's actually caused by another.

List of things I would check:


Wheels true and round without rubber (check dishing too, but unlikely to cause vibration)
Adding rubber (tire and tube) and inflating does not take wheel assembly out of true (some tires seem to float from side to side a bit and this can make things wobble a bit at speed.
Wheel attachments such as reflectors don't lead to excessive imbalance.
Accessories such as pump, water bottle, lock are firmly attached.
Check entire wheel assembly: spokes, skewers, bearings. Everything should be free of cracks, dents, bends. Parts that move should do so smoothly (bearings, freewheel). Things that should be straight should be straight (spokes, skewers).
Pawls inside freewheel engage and slip smoothly and correctly.
Frame is intact, no cracks or bends. Especially the chainstay and seat stay.
Fork is intact, no cracks or bends.
Headset is installed correctly, no play in steering.
Seat post is intact, no cracks or bends.
Seat post is tight to frame.
Saddle is tight to seat post.
Finally I would check the bottom bracket area. This gets a lot of torque and abuse so give it another look for cracks or fatigue.


Still a problem? Get an experienced rider that is comfortable descending to ride your wheel. Find a nice straight descent with no traffic.