How old are the tires? I had a Jeep with the stock Goodyear Wranglers (about 50k miles) that at about 55 mph would start to oscillate. The tires themselves looked ok, no excessive / uneven wear. When I lifted it, it made the oscillations worse. Alignment and balancing did not help. Changed the tires, problem went away. My pops put the tires on his Dakota and it does the same thing (and has for the past 6years!!) None the less, he just replaced the tires (FINALLY) and problem is gone.
Moral of the story, it could be caused by broken belts within the tire itself