I need to know if I only need to order one of these kits, or if I need to order two (one for rear driver side and one for rear passenger side)

Any help would be greatly appreciated!

Eibach Pro Alignment Kit Rear

 

That should be for the pair of arms and toe bolts. Just contact them to double-check. There are vendors that will sell just one side.

FWIW, try AutoAnything as well. They have a 15% coupon posted on their site for new customers that's valid for today. Should save you another $25 or so.