On my 1997 Chevy 1 ton dually the Steering wheel shimmies between 25 - 35 mph. After 40mph it stops. The ball joint were replaced and all the tie rod ends are tight. Other mechanics cannot figure out the problem.
Did they also check for bent rim(s), loose wheel bearings, out of round tires, flat-spots, etc? Any of these might cause a low speed shimmy. HTH.