It's not hard, but you have to have the right tools to measure the cambre. You also need to have the right new shim to put in. Most shops that work on Fords have a bucket full of shims of every description that they use to fix your truck. They drop your wrong shims in there, and pick out the right shims for your adjustment. Sometimes I have to buy one that they didn't happen to have on hand.
Make sure that the ball joints are good (#1 problem with twin I-beam suspension) and that your springs are not worn out (#2 problem). Axle pivot pushings can be a problem too, but not nearly as much as #1 and #2. If you are going through all that trouble, you might as well do the radius arm bushings as well. Get it all done, then align it once.
Springs are available from Oriellys or other parts stores, but I prefer the Ford OEM ones. They are a lot thicker metal, and they don't compress as much as MOOG. There might be other aftermarket substitutes available, but I don't know what they might be. Some of the other guys might throw out some brands for some better-than-stock ones.