1. rotate tires front to back, test drive and see if anything changes, if so change one side and narrow it down to which tire or rim is bad

2. if rotating doesn't change a thing then while driving put in neutral and see if anything changes, if so drivetrain ( if you can take it out of 4 wheel drive do it and see if anything changes )

3. also try touching brakes when shimmy starts and see if anything changes.

you just need to take one step at a time to naeeow it down

good luck!!