I would locate someone with a Hunter Road Force wheel balance. It detects out of round or flat spots with the tires. Also no rim is perfectly round and no tire is perfectly round. If you happen to match the high spot of the tire and the high spot of the rim you get a shimmy. This detects the high spots on both and guides you to offset them. In the past you could only experiment to mitigate the high spots. It can be perfectly in balance and out of round.
Hunter GSP9700 Wheel vibration Control System solves wheel vibration and tire pull problems that balancers and aligners can't fix
After that I might look at the ball joints. I think Hi-tech makes that chassis, if so the Superformance chassis have been known to have stiff ball joints which can cause what you are describing. The "difficult to turn" comment also makes me think ball joints. Cobra Valley sells upgraded ball joints you could call them and see if they are the same. I'll bet they would know.
Good luck and report back.