For load ratings click
here.
After reading that you should understand that passenger tires (P) don't get load ratings in the same way light truck (LT) tires do. LTs are rated with the A B C etc that you're thinking of.
Now you can go to BFG's website and look up the specs on the Rugged Trails
here.
Notice the LT tires have a load rating of E (example: LT265/70R17/E). That means they have a 10-ply rating and can be inflated to 80psi. Also on that chart is the Max Load, for the example tire it's 3195 pounds at 80 psi.
Odds are your truck came standard with passenger tires, probably these:
Size - P275/65R18
Speed Rating - 114T
Max Load - 2365@35
- NCSU