Here is a quick explanation:
What’s a tire ply rating? | TireBuyer.com
P, or p-metric tires are usually similar to a "B" 2-4 ply construction but are not load rated, and they are designed for passenger vehicles. The load range strength refers to how much carrying capacity the tire can hold at maximum tire inflation. In general the LT (C, D, E load) have a thicker sidewall and heavier rubber construction in order to carry a heavier load. LT tires in general are overkill for the 4runner as its weight does not require their load rating. If you are off-roading a lot, especially in rocky areas, or places that are susceptible to tire puncture then an C or E Load tire will be far less likely to fail because of their increased sidewall strength. The compromise in this is that LT tires have a rougher ride and weigh more because of their stiffer and thicker construction making them harder to balance and your gas mileage will suffer.
For your stated purposes throughout this thread I believe an LT tire would not be the best for you, or if you do decide to go with an LT tire no higher than a "C" load rating. There are many members, including myself who go off-road with P-metric tires (I had my stock tires for 30,000 miles and you can click the link in my signature to see some of the stuff I did in them). I currently run the Cooper AT3's in the "C" load rating. They have been a good compromise tire for my use, but they were harder to balance.
Here was my decision making process:
Post #122
Cymon's Weekend Warrior and MFD Thread