We need pics of your setup. Get somebody to drive your truck while you follow behind and get them to get on it hard when shifting. I've seen pinions almost point to pavement because of serious axle wrap. But if your spring pack is stiff, I think there is other factors at play here.
Sounds like you need to play with your spring pack, get a wrap bar setup, or get some springs that are designed for crawling instead of mall crawling. Rancho, superlift, and such lifts aren't designed for crawling, so they stick with stock spring lengths. Which in turn stiffens the ride, and limits flex. I bet that if you were to get some Marlin, All-Pro or ? rear springs that are longer than whats on there. Or find some 63" chevies and that will give ya some serious flex and help out with climbing. Just my 
yeah its a rough ass ride....so yeah this summer i plan on getting marlin springs, and a traction bar. thanks man=)
