check your shocks, ive had similar issues with a poor shock. i thought the guy was nuts for sugesting it, but replaced the shock and the vibration went away. (after spending the extra money to have the tires rebalanced somewhere else, because i thought the guys that did it the first time might not have known what they were doing. turns out they were right and i was wrong for doubting them). hope this experience helps. Let me know what you eventually find out.
"Let me try it again! I think i can get another 10 feet of air!"
1991 extracab 2wd,full cage front to back,camburg suspension,sway away coilovers,fiberglass bedsides
,fiberglass onepiece front,33 inch tires,4.56 geared 9 inch,3.0 motor w/5 speed(soon 5.0 and c4),corbeau suspension seats,4 point harness,fuel cell,custom dash,almost 1k lbs lighter than stock,NO LICENSE PLATES!!!!!