Well, you'll want to allow it to shorten as the springs compress, so you don't want it too tight in the resting position. How long it needs to go depends on your suspension set up - if you have springs and bushes that allow the axles to droop very low then you'll want something on which the spline will stay engaged at maximum suspension travel. Most wide angle props are designed to accommodate this sort of thing. You want something that will squash up an inch or two shorter than the distance between the drive flanges when the vehicle is at rest, otherwise when the springs compress it will put a lot of pressure on the bearings in the diff and transfer case.