A follow up on advice given, and some advice for anyone buildling their own cab right now...
Some time back in the summer, I mentioned to Drew and several others how my cabinet seemed to move side to side a little too freely. I was told some give was normal, but bracing it again wouldn't hurt. I believe, in fact, it was Drew (might have been someone else) who told me about doing an "X brace with 2 x 4's" under the monitor shelf.
To cut to the chase, my wife and I finally had time to brace it today. We feel MUCH better about the safety of the cabinet overall (we added two front vertical braces to the monitor shelf and placed the X brace in the back) as it cut the "wobbles" at least by half. It still moves a little, as it should, but it was a point earlier where the whole cab would shake if you slammed a joystick too hard. Was afraid it would just fall apart, and we both knew we'd cry hard had that happened!
Anyway, thanks for the advice!