Answer: I spoke to Rancho's technical department, and the reason that none of their parts are designed to fit with the snow plow package is that it came with stiffer springs, which the ranchos weren't designed for. He did tell me that it isn't a fitment issue, just a valving one, so I would recommend using a high pressure monotube shock that will be valved stiffer than the Rancho units to compensate for the extra spring rate.
You can see a few shocks that I would recommend here:
KYB Monomax Front 1998 Dodge Ram 2500 4x4
KYB Monomax Rear 1998 Dodge Ram 2500 4x4
Bilstein Heavy Duty Front 1998 Dodge Ram 2500 4x4
Bilstein Heavy Duty Rear 1998 Dodge Ram 2500 4x4