[cordenj]

Originally Posted by Phil Waterman

Given my experience would try the following with the wheels jacked up take large block position so it just doesn't touch the tire hold something on the block and slowly turn the wheels to mark a line down the center of the tire. Then look at your line is it staying centered on the tread and did then your marker try to move in and out as you turned the wheel. Assuming the tires are running true an are not egg shaped, then would try moving your tires around front to rear side to side, does the shimmy change?

Thanks Phil,
I did wonder if changing the tyres was partly the cause. The ones on it when I bought the truck last autumn were very badly cracked - so I sourced some good secondhand 11x20s from a Bedford RL. But didn't get to drive it very fast with the old tyres, so I don't know if the shimmer was there.

I'd have hoped that the professional balancing of the front wheels/tyres would have highlit any defects, but I'll try your recommendation.
I might also take up a offer to try some other wheels with 12x20s fitted to see if that cures it.