I have the 275/65/18 Michelin ltx m/s2's on my truck currently. as my truck is also my commuter to work, I stuck with the P rated for the comfort and better fuel economy. This was the stock size on my Limited, usually just bump my tire pressures up to max while towing. have 20K miles on them currently and are wearing very well. Got them from discount tire with the repair/replace contract. So far towing has been fine, and considerably better than the stock Bridgestone Dueler's it came with. (which were worn right out at 32k)
I was also looking at the hankook dynapro's, but the michelins at the time were about the same money and had a longer warrenty on them.
as to Q1, i have had nexen tires from walmart on a few cars (not truck) in the past, and although I didn't have any major failures, i did find they were noisy and their traction to be lacking.
Q2: i think has been already answered.
Sticking with an O.E. size won't give you any headaches with rubbing/speedo issues. I was looking at the smaller size for a set of winter tires, but i think they are a bit of an off size, as the larger ones were cheaper at the time than the ones you currently have. (F150 has the 275/65/18's on it as well)