BJ pretty much covered what I would say.

I hate having tires replaced. Never a cheap thing to do.... But every time I get new tires, all the vibrations and little wiggles seem to go away.

Check them for balance and true spin. I'm lucky enough to have a store at the corner (only store within 30 minutes) that has a wheel balanceing rack outside that is available any time.... This will tell you if your wheel/tire is balanced, but won't give you the spin deal to see if there is any wobble. shade-twig way of checking this is would be put all four corners up on jack stands on a good surface, crank her up and idle in first and second gears, have second set of eyes on the ground watching the wheels through various ranges of speeds. I seriously wouldn't get up to more than 20 MPH or you might wind up in a pond or through a wall if the stands aren't that sturdy.

Clean the wheels insides. I know my front ones have a nasty tendancy to collect goo from the lube points and dirt/sand/mud from the roads around here.... Rear isn't much better, just no grease to attract even more stuff....

This could be two tires that act strange at different speeds. Could be a loose suspension component. Could be a bad bushing or shock....

Only questions other than the contents of the insides of your rims would be... does this happen at all positions of your steering or just straight rolling? Does it happen only in 2WD or also on 4WD?

Good luck and keep us posted!