Maybe the tires are mis balanced. Check to see if the weights are missing from the rims. The tire could also be "out of round" or have broken belts. Bad shock could also cause the same issue.
'99 WJ, 4.7L, completely bone stock, except for 245/65/16 tires
'76 Jeep Grand Waggy, 360cid Edle 660cfm/edle intake Quadratrac
'02 Exterra 3.3L, 5-spd, traded YJ for it :(
'89 YJ 2.5L, 5-spd, Borla header, dynomax super turbo muffler, Big-O XT 30x9.5r15
2004 Jeep Liberty Renagade 3.8L 2wd
'86 XJ 2.8L V6, 5-spd manual tranny (My 1st)