The first thiing I'd try is rotating my tires. If the shake moves, then it'* a good bet that a rim is bent or tire tread is starting to separate.
If the tire is starting to separate its tread, usually the car will 'waddle' at low speeds. Oddly, I've noticed that tread separation vibration peaks around 45-55 and then decreases at higher speeds.
Sometimes, tire cords will start to do weird things inside the tire and the tread won't bulge. I had a tire shop tell me a story like that... they had the company reps come in and everything. Finally, they isolated the bad tire by moving the tires around on the car and then replaced it.
As Bill said tho, could be many different things causing your trouble. This is only one suggestion. I've had more than my share of tires separate on me. Not sure why, but mostly due to age. I don't wear them out fast enough I guess.