Also, tires that are cupped will cause it. Bad ball joints or bent tie rod can contribute to a "shimmy", as well as wheels out of alignment. All of these along with the aforementioned tire balancing can contribute to a "shimmy". Interestingly enough my son's '05 TJ recently suffered from this shimmy at around 50mph. Sometimes it didn't shake, but most times it did. He had all these problems. A bad pair of ball joints on the drivers side wheel (lower ball joint actually bent and had to be cut off), the tie rod was bent, new installed tires (off his S-10) weren't balanced properly, and the passenger side front sway bar link was broken. Fixed the sway bar link and rebalanced the tires. Helped but still shook. Replaced the tie rod with a ZJ conversion. Helped some more. The repair shop noticed that the tires were cupped, so we tried rotating them, but that didn't help. We took the tires from my TJ and swapped to his...shimmy was gone. So he bought a new set of BFGs and all was good. Almost, the repair shop also noted the problem with the lower ball joint, so he had to have the drivers side ball joints replaced and the front end aligned. Drives great now. No shakes or shimmies no more.