20" LT-metric E-rated tires are really, really expensive, if you can find ones that fit. Not too great a selection. Found some LT285/55R20E for $440 ea.
I'm picking up some 17" take-offs this week and putting on Michelin LTX MS/2, LT245/75R17E, the cheapest size of that tire I could find for the truck. $270 ea is more like it.
Will sell the old 20" wheels and tires to cover much of the cost. Looks bedamned.