Ernie is right!
"P" is the designation for Passenger tires. "LT" is the designation for Light Truck tires.
LT will give a harsher ride, but will generally carry a heavier load. However, there is more to it than this.
Check the tire rating label on the driver door. Note the GAWR, or Gross Axle Weight Rating. Generally, the rear GAWR is higher. Divide the rear GAWR by 2 and make sure the Load Ratings on the tires you buy exceed this number. (2 tires per axle). Tires also come li "Load Range C", "Load Range D", etc. Higher letters correspond to more plys or ply ratings, thus heavier tires with higher load ratings.
Note the rim max pressure designation as well. Higher load range frequently have higher pressure requirements to reach max load.
Note also that more tire capacity does NOT increase the load carrying capacity of the truck. You still have the same axles, brakes, suspension ...