Standard shafts don't usually have more than 2.5-3" of slip in them. Is that enough? Well figuring that out is part of building a custom suspension.
Simple trigonometry is all that's needed to figure out how long your shaft should be at stuff, droop, etc. The amount of slip needed is proportional to the amount of suspension travel available. The problem is that adding lift doesn't mean that a longer driveshaft with the same amout of slip will work. Unless you space down the bump stops or limit the stuff somehow, the minimum driveshaft length is the same as stock.
You pretty much have to design it for full compression, so that you don't bottom out the driveshaft into the T-case on stuff. Then you put in as much slip length as you can afford. It's OK to overextend a little and booger up the seal, as long as you still have enough spline engagement that it doesn't come apart or bend the splines.
As for moving the axle backwards, that should translate directly to a longer length needed.
