Oscillating vibration

[viper5b]

ive got a 97 f250 crew cab swb PSD auto 2WD. bought it 2 weeks ago and its been very well kept. i got it stuck a week ago. friend rocked in back and forth until it came out. before getting stuck, it floated down the road beautifully. that night and next day i noticed some vibration around 55. figured i had mud caked. i washed all mud out and washed inside rims and nothing changed. over the past couple days its gotten worse. on smooth road i can feel a slight bounce/wobble around 30 and pretty violent shaking around 55 and up. its mostly in seats, sometimes i think i can feel it in the steering wheel. funny things is on smooth road itll shake for 30sec-2mins the go 100% smooth for 1-3mins...just keeps cycling like that. nothing affects it. i hit brakes, i threw in neutral, i stomped on it, jerked wheel left and right...none of that affected it....yesterday had ujoint by rear diff changed cuz it was a lil tight, had carrier bearing changed, had 1 tire replaced, had all tires balanced and checked for problems/seperation, also checked front end for slack when tires were off...had a little up and down movement on both sides but not much...ive only had truck 2 weeks....anyone have any ideas?

[bigjoe]

Did you check side to side movement for a bad tie rod. Joe