I had the same trouble on my T-bucket. I raised the air pressure to 30 lbs. (I was running about 15), re-tightened the wheel bearings, checked all the drag link, and tie rod connections, DECREASED the caster to aout 4 degrees, set the toe-in to 1/8" and it went away. Don't know which one of them it was, or if it was a combination of them. I've also heard that the toe-in will vary depending on whether you are running radial or bias-ply tires.
Hope this helps, shimmy isn't fun!!