What have you tried for ballast in the truck bed, and how are you distributing weight on the trailer? It sounds like your tongue weight gets lower after you load the trailer, or more specifically when the forces change from the horizontal to the incline. I'd try getting more weight in front of the front axle on the trailer and/or getting weight in the truck bed over the rear tires before I looked at retooling the set-up. That said, there are some tough roads, curves, and situations out there.
Goosenecks advantage is supposed to be the superior weight distribution across all tires, where bumper pull without load leveler levers weight off the front axle and adds to the rear. Your description sounds like the loaded trailer may be removing weight from the trucks rear axle, or that the roadbed is too loose to stay put. 4wd is great for snow, but I've never used it towing--I have a lot of tongue weight on my bumper pull, but its only a 20ft trailer, and I don't tend to have more than 6000# on it, on a 2007 4wd f350... I guess the load with trailer may be up over 8000#. Does have the heavy 6.0 over the front truck axle which helps me feel better about getting more weight in front of the front trailer axle without lightening the front of the truck much.
I'd get some weight in the truck itself and see if that can overcome your moment when the load exceeds the traction on the grade... if its the roadbed that's giving out rather than the set-up some refreshing might be the easiest cheapest solution.
