With your suspension bottomed out, you should still have some splines left for your driveshaft to go into your transmission. If I remember right, at normal ride height you should have 3/4" of splined length sticking out of the transmission, but that all changes with lift, lowering, modified suspension travel, cranked keys, etc. The bottom line is that if you take the springs out, set the suspension all the way down, and unbolt the driveshaft caps from the differential, you should still be able to move the driveshaft in towards the transmission a 1/4" or so. At full height it should have enough spline engagement to not strip the splines nor for the driveshaft to fall out.