Not hard if you have a large vise or access to a press, you need to press them in and out. I recommend having it balanced as well. I took mine to a shop and they balanced it and installed 2 new joints for $150. Search for driveshaft repair in your area for shop.
Jeffksf is correct...It's pretty easy to replace the U-Joints. It only requires a large vice or large C-Clamp and an assortment of sockets. The sockets are used in conjuction whith the vice to press the bearing cups in and out of the yokes. You will need a socket larger than the bearing cup and one slightly smaller than the bearing cup.
You should not have to have the balance checked on the driveshaft unless you have been experiencing driveline vibrations. The key is to mark (magic marker works fine) the yoke and the driveshaft at each end so that each yoke is reassembled in the exact same relationship to the driveshaft, and you should not have any driveshaft balance issues.
Spend the $$ for heavy duty joints. With grease fittings. Pump some grease in before installing the shaft. Most of the time the grease gun head. Wont fit after installing the shaft.
Also have a torch ready.......Or a really big breaker bar! I had to take mine to my friends shop and have mine taken off. I tried to remove them myself but I was just lifting myself off the ground trying.....and I'm not a small guy! Ford used yellow lock tight, I've been told, and it is extremely strong and normally needs to be heated up to be removed. Good luck with it though!!