Hello Ron,
Similar question/advice was posted in an earlier thread. Nonetheless:
1) Will a new steering gear box likely help?
1. Maybe. Have you adjusted the steering as described in the Shop Manual? How good are your other front-end parts?
2) Is gear box replacement a difficult job? Im an average home mechanic with no fancy tools.
2. You'd most likely have to clean off a lot of grease and unbolt about 4 large bolts between steering box and frame, and remove the steering wheel, and remove the bolts/clamp that hold the steering column to the dash. The procedure is described in the Shop Manual. A rebuilt steering gear assembly would cost around $350.
3) I hear about king pins creating steering trouble - is that more often a problem with these trucks?
3. More often than what? King pins wear out and are commonly found to be loose in these old trucks, which were poorly maintained after their first 10 years of existence/service. This is easy to diagnose (Shop Manual). If the axle cannot be serviced and/or the steering knuckles/spindles cannot be serviced, this can be a challenging repair.
Do you have tools and experience in doing mechanical work on cars/trucks?
Do you know a garage that can/will work on an old vehicle?
Do you have a Shop Manual?