I doubt it is the tires (possible, but doubtful). The Spec-X is a respected tire and no one has really reported any problems with them on the FX. I bought a brand new set of tires after thinking my Toyos were the culprit only to be proven wrong. Luckily I kept the Toyos and can utilize them to some degree if/when I figure out the cause of my shimmy. I am betting your installer just does not know how to balance, or did not properly prep the wheel surface for the stick-on weights and they ended up falling off (this seems to happen to me every time I have a balance)...sometimes you can hear it, like a rock being thrown around in the wheel well.6t8stang said:I'm working the same issue here as well - also an AWD. Started about 1000 miles after getting the Spec-X's installed. Had the tires balance twice since (at the same place) and both times were out of balance. I opened a case with Yokohama and have the tire place talking to them for next steps.
A wheel shimmy with no known cause seems to be an increasing complaint among FX owners. Some do have balance issues whereas others have never found the cause or it took months to finally track it down (such as one owner finally had a mechanic realize a washer was missing from the steering rack).