There is no law that says you must replace everything at once. If the cam is still ok, no flat spots and no excessive pitting, just replace the lifters that are sticking. If the engine has a lot of miles on it you'll probably be replacing/rebuilding it soon anyway. Replacing the lifters will just give you a few more miles before you need to get a new engine or a new pickup until something else fails. If you have a flat spot or excessive pitting on the cam, new lifters won't do much good.