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Did you recently have a brake job done? If so then the problem is that when the parts were put back they weren't cleaned well enough. I used to have problems with my vehicles all the time after doing a brake job. It slowly gets worse over a couple of months. Then the last time I did them on my Taurus the shop manual said to make sure all metal-to-metal mating surfaces were void of debris. I didn't pay much attention to that statement and sure enough my front end starts to shake. Then I took everything back apart and blew them clean with compressed air and took a steel brush to every mating surface until they were shiny clean. That made the problem go away. I think that all mating surfaces have to be clean because all the rolling parts are machined to line up straight and the slightest pebble of sand can knock out the alignment.
I had the same occasional problem on my 93', it was a bad tire. Since you rotated the tires, that would eliminate that. I'd jack up the front end, remove the tires and brake caliper and check for bad hub bearings, then ceck tie rod ends, etc. If it wobbles when braking, check for warped rotors.
Well I found the problem. I rotate tires every 12k miles. Ended up with a plugged tire up front. The tire was separating inside. NEVER plug a tire, I found out. It is illegal in thirteen states now. Always patch from within. It may cost a little more but you should see the plugged tire from inside. They look GREAT on the outside. With three kids always on board and pulling horses, I have learned a valuable lesson about plugging tires. PASS IT ON.....