[jakeru]

Wheel bent or tire out of round, gives some oscillation movement, mostly noticeable at lower speeds. A wheel out-of-balance condition will typically only show up as bad vibrations at higher speeds (and will worsen as the speeds increase).

Maybe start by making sure your lug bolts are all evenly torqued, and the wheel-to-hub surfaces are all clean?

Might try jacking up each corner, one and a time, also giving each wheel a spin, and visually observing to make sure the rim and tire doen't have gross run-out. If you identify one wheel with run-out, you've probably found your culprit. While you are at it, check your tire pressures, too. All this stuff is relatively easy to do.

Good luck!