That box is a Saginaw 525.
It's simple as simple can be. You can buy a rebuild kit for $60 or so, sounds like it needs the whole kit.

As it's a recirculating ball type the interior parts should be all good, with exception of the output bushing and the bearing cages. It'll take longer to get it degreases than it'll take to rebuild. I don't know how to get the output shaft bushing out, I rebuilt mine in the middle of a public parking lot and didn't see the need.
I used a Moly NLGI 1 grease, left the tube in the sun and it just sort of slid/poured into the case. Kingpins will always need replacing, get the kind with the bronze bushings. remember to take out the lock pins before attempting to drive out the kingpins. The kingpins are relatively soft and you can mushroom the end if you're not careful. Spindles will need to go to machine shop for press out press in and ream.
Tie rod ends are cheap, and the drag link isn't but will probably need to be replaced. That's all there is to it!

R.