For the shocks, I used a ramp to determine the fully extended and compressed measurements and determined the shock number from there off of Rancho's website. You can also adjust those measurements just a little with the shock towers to accomadate the shock length. If you don't have a ramp, look for a loading ramp, or a set of stair, or a bank that will extend out your suspension. I drove down the street without shocks and it wasn't that bad for a few blocks.
As for driveshafts, this is what I did. The rear is fairly simple. I determined the maximum distance the diff would travel and the measurement from the T/C to the pinion when the truck was level and normal ride height. After a few questions from the driveline shop, we determined that I didn't need a long travel spline (due to the rear driveshaft is quite long anyway) and went with a Double Cardin joint at the T/C just for extra ride comfort (I drive 85mph on the freeway everyday). You really don't want a long travel spline in the rear due to vibration. Their tollerances are not as tight as the standard splines, thereby vibrating. Now the front is another story (obviously by the diff being on one side). Here I had to have the truck on a ramp and determine the lengths all the way through the suspension travel to determine the length of driveshaft and what length of splines needed. If you don't have wicked suspension travel, the you might be able to get away with normal splines. Each rig differs.