Bad ground strap, worn brushes, and wiring not correct = bad. Not something I would want to trust on a weekend when I was trying to relax and drive my old truck. But you already know this. When that truck was new the guy at the garage knew how to rebuild it and maybe even had the tools to test it. I taught myself to use those old tools when I got a job in an old dealership that head 'em buried in the back room. I had to find a few old books and bug an old timer when I had questions but over the years I've even picked up enough to know when that $3000 machine at the store doesn't know all it sees.
Today's starters are built differently and it's cheaper to buy new than to rebuild. It's tougher to find a place that can do the job since they've been replaced with Auto Parts Retailers. Maybe you've got a starter and alternator shop near you? A place you can walk into and talk with a guy, look around, and get real answers? I've got one near me and I try to do enough business with them to help them stay open.
Your starter is a 10MT, part number 10455305. Well, at least the end cover is. I don't know if you can still buy "good" parts for overhauling them. If I needed parts I'd probably call my favorite shop and ask 'em for what I needed. However, this site seems to have a good collection of rebuild parts for the 10MT:
https://www.aspwholesale.com/starter....html?page=all
Good luck.