There is a nylon (?) sleeve that was between the two parts of the steering shaft that you have pictured when these trucks were new. They don't seem to still be in place these days. You'll never get the slop out of the steering without addressing this. You can replace it with a OE style shaft that will probably have the same issue some day. There's two upgraded options that I'm aware of for this and both are made by Borgeson (I think). There's a two piece, OE style and a one piece that won't collapse in a front end collision like the OE one does. You can also "cheat" like I did. I drilled and tapped a hole in the outer piece of the OE shaft. The I put a bolt into the hole to hold the inner shaft tight against the side of the outer one. I used blue Lock Tite to hold it in place. I also didn't tighten it very tightly, just enough to hold the inner shaft snugly. This will still the two parts slide (collapse) in a front end collision. Here's my example.
