If you're getting a wobble in your rear wheel then 1 of 2 things is wrong:

1) Your rear wheel isn't straight in the dropouts (or the frame/drops aren't straight with the frame alignment)
2) Your bearings aren't properly adjusted (cup/cone) or are worn out really bad (cartridge) and the wheel can get a wobble.

Speed wobbles happen on the front wheel due to geometry issues having to do with rake and trail ratios. Wobbles on the rear shouldn't happen, regardless of geometry. That's why you can have a touring frame with super long chainstays which is just as stable on a mountain descent as a tri-bike with a forward cut-out designed seatpost that makes the effective stay length shorter than the radius of the wheel.

(Someone correct me if I'm wrong about this.)