Renegade,
You likely have the 5.13 gear in it. 4.57 gears are easy to find. 4.10 gears are a bit difficult to find. 3.90 gears are scarce as hens teeth. I once heard a rumor about a mythical 3.38 gear. If you can find a 4.10 "third member", buy it and you'll at least be able to drive 60 mph without burning up the engine. Swapping out third members on these axles is a piece of cake.
I don't have any recomendations for a rear swap, except to go to a reputable yard and start looking. A GM 14 bolt is a good strong axle that will accept a load that a 3/4 ton truck might need to deliver.
Make sure you find a new axle with the same stud pattern or you have to buy new wheels. Your wheels likely have 1/2" stud holes, while newer ones will have 9/16" holes, be ready to drill. Are your wheels stud piloted or hub piloted?
I'd be scared to pull too much with your truck. That sounds like a lot of trailer and load to haul. A 3.50 gear will really reduce your torque. If your 235 is stock and worn out, it won't have excessive torque to begin with. I'd be at least as concerned about your ability to stop that load. How good are your brakes?
