A friend and I were talking about the issue of gear ratio and 8-speed transmissions recently. Our conclusion was that the axle ratio doesn't matter as much with more speeds, because you can just limit what it uses as high gear. You may effectively only use 6 of the 8 most of the time, but even at 6, that's much better than when you only have 4, with a steep OD ratio, like we used to have. Even with my Dmax, when I tow, I usually limit my truck to 5th instead of 6th. Keeps shifting to a minimum, and usually results in the same mileage, without putting so much stress on the engine/trans.
That said, I will side with p nut that putting a larger tire on a truck with a V6 and 3.21 axles seems a little counter-productive in terms of power and efficiency... I also think you may be shooting a bit high on tire size.
Limit the size bump to one over stock, and put a very mild lift (1-2") on it. That'll keep the speedo fairly accurate, and allow you to retain the stock wheels without rubbing, which saves you $$ and keeps the truck driving nice. If you have a rear LSD, it'll also be more effective with a tire closer to stock size, as will stability and traction control.
Use tire pressure to your advantage when the going gets soft. Even with the OE size tires, if you air down properly and drive smart, you're VERY unlikely to get stuck. (Driving smart pretty much means not getting it crossed up in sand. Keep to the more level route.) When you get back to the highway, air up. A little $30 "high volume" compressor from HF has served me well for several years now. (To save time, I don't usually air all the way up, I just air up to a safe level, and use a real compressor at the next opportunity.)
If you use the truck for a while and find that you really do need bigger tires, it may also be apparent at that time that your v6 Ram isn't the right vehicle anyway, and then you can move to something more appropriate for more difficult offroad use. ??
My $.02.
Chris
