this is gonna happen no matter what. the rear axle pivots around a fixed point as it is hooked to the driveshaft. This means that if you "lift" the rear of the truck, the axle is not going to just go straight down unless you get a longer driveshaft. Think of this: if you compress the rear suspension, where do you think the axle will go? Straight up? No. Its gonna swing up and back like its hooked to a pendulum (ie the driveshaft!) Does that make sense? Try this out: take a ball, tie a string to it and hold it level. This will represent the rear pumpkin and driveshat. Now hold it horizantal and lower the ball (equivelent of lifting the truck) and see where the ball goes. It doesnt move straight down! it goes down and forward following the arc created by the fixed driveshaft. I hope that makes sense... Ive seen youre other posts about this so I hope I have cleaerd this issue up with you. Its just the geometry of lifting the rear of a truck, its normal. Only way for this not to happen is a body lift!
As for the driveshaft shaking in lower gears, I would check the pinion angle as maybe the shims (those little wedges) which should have come on the bottom of the leafsprings are pointed the wrong way. They should be pointed forward. front of truck < rear of truck. This lines up the pinion correctly.
I have the exact same set up and no problem. you can see my axle has moved forward as a result of the lift:
(maybe hard to see in this pic, but its the best pic i have)