Does anyone have any suggestions on what I can do to eliminate or at least reduce the bottoming out of my suspension when I'm carrying a passenger?

Crank up the rear springs...should be simple coil-over design; probably w/ fox rear shocks...& rebuild the shocks if you havn't already.

-Or you could always go to heavier rear springs-

What will it do if I add preload to the front shock on the track? (the rear shock is not a coil over) Will this help? Who sells heavy duty springs if this is what I need to change ( I'm already at the highest point on the spring blocks)

The rear springs are ten year old. Replace them, and make sure they are for a two up. I think that should be your first step. And buy the ones from your Polaris dealer. Also tell them the weight of you and your passenger as they have different rate springs for different loads.