I agree with del1650 on making sure everything else is in good shape before even trying this, and also of the potential downsides of over-tightening -
Suggest that this steering box adjustment is just a partial fix or patch. It can take some of the slop out, but don't expect it to be 'as good as new' if the truck has a lot of miles and the steering box subsequently has a lot of wear.
Tighten no more than 1/8 turn at a time, then take a good test drive. If you get it too tight, the steering will tend not to return to center after turns - it will stay where it's put, which can be dangerous in turns if one isn't expecting this behavior. Also, you will end up correcting the steering going straight on down the road more so than the way things started out, if you tighten too much. If you tighten upwards of a 1/2 turn or more - probably gone too far, and will not get any additional improvement with further tightening.
Steering box - look in the engine compartment down the driver side of the radiator, where the steering shaft from the cab ends up. There will be a 'box' with a slotted screw held by a nut. Loosen the nut about 1/4-1/2 turn, then tighten the slotted screw. Hold the slotted screw in place, and re-tighten the nut.
Some others say they've had great success with this - I only noticed a marginal improvement, thus my lack of enthusiasm about this procedure.