The units you buy from parts stores are most likely rebuilds. And most of the time they're garbage. This is why I know so much about this subject. I'm on my 3rd steering gear since replacing the OEM 2 years ago, and this one is going haywire. The very first one I put in leaked fluid all over the side of my truck and was out within a week. The next was when I stripped the splines on the end. Now this one loses power steering at low engine speeds (think, backing into a parking spot... its obnoxious). You may get a better rebuild out of a shop that does them, as long as you can stand being out of a truck while they're workin on it. Look in the phone book, theres places all over that do it. Odds are you're best suited to pull it off yourself and take it in. It may cut down on lead time, and most likely will cut down on cost.
If you pull it off yourself and you think your tires will be in the way, put the drivers side on a jack stand and pull the tire off.
The screw is right on the top slightly forward of the lines. I've never used it, I just know its there and I know for fact its an adjustment on how tight those gears are to each other. I would imagine that righty tighty should take slop out. Move it a little at a time and test.
If you do pull the steering gear, I'd replace or weld the plate. Like I said, I got one for $20 from the dealer, no mess, no fuss. The other problem you encounter is the holes on the plate can oval out, causing play, so you might be better suited with a new part. The crack itself may be where your play comes from. If its cracked, the gearbox will flex back and forth as torque is applied either from your steering wheel or from the ground... which also means that crack is only going to get bigger over time.
Be ready to fight the pitman arm to get it off... its a pain. Try a puller, but I bent mine and the stupid thing still didnt come off. I was in the middle of replacing most steering components to fix play in the steering (sound familiar?) so I just bought a new pitman arm ($80), a new draglink ($60), and went right down to the tie rod ends ($60 each). When all was said and done, the coupler was the biggest impact, even though it seems like its your steering gear, most times its the coupler if its OEM. Start with boregson shaft and work your way down... you may be surprised.