on my truck i replaced it twice, then ended up replacing the steering box,
Heres what i personnaly recomend,
Remove the nut on the pitman arm you will need a pitman arm puller to remove the arm it self, if you dont have one, there around 15-20$ to get,
Then there should be a dust cover, i thing, and a snap ring, remove that and then the actually seal is a bugger to get out, put a catch pan underneath and fire the truck up, and bump the steering agaisnt the stop and this should get the seal to come out, at this time, inspect the groove for the snap ring, if it looks worn, like mine was, i suggest replacing the entire box, thats what i did in mine, it didnt hold the seal the 1st time, ended up getting a replacement steering box, for 60$ at the u pull, very common and easy to find almost all the ford trucks have the same box from late 80;s to mid 90's
Hope this helps