I once had a similar problem on a 58' 1/2 ton. The spindle bolts and tie rod ends were in good shape. The shimmy would only show up after hitting a chug hole or some other type of bump.
I took it to a front end shop and they balanced the front wheels. I didn't think it would help but it did solve the problem. Funny thing is, there was no usual indication of an imbalance.
meansam
