Scott emailed me this solution for the wobble:

Place the unit on a flat, even surface. Loosen each of the six screws (three on each side) that hold the top cover on the unit. It only takes a turn or two. Press down on all four corners at the same time (forearms work best). Then retighten the screws.

I had four that wobbled to varying degrees. On three of them, I didn't even need to push down to straighten them out. When I loosened the screws, the weight of the unit was sufficient to flatten it out. On one, I held my left forearm diagonally between the two corners that rocked and retightened the screws with my arm in that position. This has completely stopped any wobble. The units are rock steady now.

It appears that when the thing was assembled, it was either on an uneven surface, or held in some sort of vice that caused a slight warp. When the top was attached, the screws froze the unit in that warped position. But, that's just a guess.