I've never done a boom cylinder, that will be a big piece of iron to handle. Definitely not going to lift that out by hand. It might be easiest to dig a trench and point it straight down, use another machine to lift the rod out of the cylinder, and put it back in. Is that a bolt on gland on the boom cylinder? with JD's funny locknut style gland on all the other cylinders. That will be the easy part. The tough part will be breaking the piston nut loose, guessing around 2,000 ft/lbs to break it loose. There are some threads on here about cutting a wrench out of plate or similar ways to tackle a nut like that if you're game to try it. Otherwise you can take the rod and gland to a hydraulic shop to take apart, or the whole cylinder if you can manage to get it out of there.