I am not trying to increase my speed, I am trying to get into the powerband of my truck at a lower speed. Currently it pulles the trailer great at just over 70mph, I want to run the same rpm at 60mph. Fuel mileage is a non issue. I only drove the truck 1500 miles last year, and get 7mpg towing, it will not get much worse, I rarly run empty and don't mind buying the gas when I have to. I pull the RV to go have fun, not to save money. I just hate it when I hit an incline and lose speed till I can downshift to third, only to have to shift right back to 4th when my rpm and speed get back up.
All that being said, swapping the rear end and getting parts from the salvage yard sounds like fun to me, so why not? If I don't like it, I will just put the old rear end back end and sell the one I bought at the salvage yard.