i'm brand new to diesels, just got an 89 f250 extcab, 4x4, 5spd, 7.3l all stock with 236k on the clock. i put about 200 miles on it, then it started acting up. as i'm decelerating, at about 1500 rpms and just let off the accel., the truck started to shimmy pretty rough until i hit the accel...