Proper pinion preload is with only the pinion installed with all seals and shims, the proper preload is 15 to 20 inch pounds with old bearings and 20 to 25 with new bearings, right from the ford service manual, proper backlash is just with the carrier installed, not with the axles, spider gears have quite a bit of play in them and the backlash should be .08. The most important is to always remember dont go to tight, As the rear heats up everything will expand, causing binding, then destruction. I always favor the loose end and always check your pattern with the dykem that is supplied with your gears or carrier. After you coat acouple of teeth on the ring gear spin the yoke and keep a proload on the carrier with your other hand. MAKE SURE that wear pattern is in the center of the gear, I always use mockup bearings, old diff and pinion bearings that I hone out so they slide easy on and off, your rear the carrier shims are on the outside of the bearing so you dont have to worry about that, but with pinion depth you may have to try 2 or 3 times to get that right and pressing off a brand new bearing, no matter how good of a bearing seperator and press you have, damage does occur. Good luck remember a little lose wont hurt.
IF your using the original pinion and bevel gear, then measure the shims that came out and use that dimension for the new. That will be real close. If your installing a new pinion and bevel, the numbers may be stamped on the end of the gear or the directions may recommend a shim pack to start.... If the rear end is out of the truck, install the pinion and point it straight up ( no bull shaft or gear). Easiest to check preload when rotating the wrench horizontal. Tighten the nut while checking end shake by pushing on the pinion gear end. When the clearance gets small, start going in small increments and checking drag often (proload). I used 25 inch pounds with new bearings, old gears. .010 backlash is a good average.