Hi Ronda,
I'm not sure there would be any really good sub for IMAX2 that wouldn't also bother your knees. LowMax would probably be perfect, but not this month, I guess! If your knees are the problem, then just pick the most intense cardio workout in your collection that doesn't aggravate your knees. The idea is to give yourself a cardio challenge. Alternatively, if there are specific IMAX2 moves that bother your knees, post them here and I'll bet the educated crowd could suggest some modifications.
Cheers,
Sandra