Depending on whaether you're looking at P rated/P rated XL or LT rated tires as to what the ply ratings are. The size will determine what ply ratings are available. Unfortunatly, tall and narrow is somewhat of a rare bird in the tire world these days, especially in a 15" radial.
First, I suppose some explaination of ply ratings is in order. You mention the 8.20-15s from Firestone and BF Goodrich that are 2 ply. In this case, those are the old bias ply tires. The ply rating is simply the number of fabric plys (originally cotton, later polyester and even fiberglareinforced polyester) that the tire had. More plys=stronger/heaveir load rated tire. In todays tires, the "ply rating" is really more of a "ply equivalent", as the tire itself may not actually have the same number of plys as the bias equivalent.
Now, on the ratings. The 2 ply tires mentioned are not for truck use, strictly passenger. For the most part, trucks use at least a 4 ply tire on 1/2 tons, with 6 ply optional. Depending on year, heavier trucks can get into 10 and 12 ply tires, depending on size. Whith todays tires, the lower ratings are enerally taken care of in the P-Metric tires, with a P rating being 3-4 ply, depending on size (larger tires tend ot be 4 ply). There is also a 5 ply, which is the P-XL, almost always a 235/75/15, though there are some P rated light truck tires in 16" and 17" that are coming in XL ratings, too. The LT tires use a different rating system, though still based on ply equivelent ratings, they use a letter system to designate the rating, called "load range" in tire terms. For light truck tires, for the most part you will see "C"-"G" ranges, "A" and "B" are used mainly for light trailer use, and the "B" rated truck tire is obsolete, havng been replaced with the heavier P/XL tires. The low end "C" range is a 6 ply tire, with each letter after being 2 plys higher rating. Most light trucks stop at "E", the heaveir range tires start getting into 1 1/2 ton trucks and also 19.5s.
Now for what you have. With a 15" rim, while there are many choices, tall and narrow are not really one of them. 235/75/15 is about the largest available without going into floatation sizes, which start really getting into wider tires in addition to taller. If you want tall and narrow, you're alomost going to have to go to a 16" rim, using either a 215/85/16 or 235/85/16.
<y personal suggestion, go with the 235/75/15, preferrably in a P/XL or LT range C. That will handle the load of a truck without loosing the ride quality, and you will still be able to use it to haul stuff in needed. If you don't mind a little stiffer steering, a 30x9.50x15 C range floatation tire would be another choice. That will get you a bit taller tire than the 235/75/15, but not really get too wide. Go for a highway type tread, I'm partial to Michelin LTX/MS myself. ALso, you want to make sure you have the proper wheel to handle the stesses of not only a radial tire, but also the give proper support to the tire. A 7" rim is fine for the 235s, an 8" is better for the 30x9.50.
