Just start at page 29 (adjustments) in the manual I linked, all gear boxes are pretty much the same when it comes to adjusting.
What is in the gear box now? nasty watered down greasy oily crap, or decent looking gear oil? That should determine if you need to clean it out or not. Pull the side cover and rinse it out with solvent and brushes if you don't want to pull it all the way out.
If your truck set out in the weather or had no windshield, water can get into the gear boxes. There is no seal at the steering shaft so water can run down the steering column tube right into the gear box. I had to go through four boxes to find one with out water damage to lower bearing when I was working on mine.
Joe
