...The box was from a 77 I believe, but as stated earlier, it was rebuilt professionally.
A '76 or '77 box would have about 4 to 4.5 turns lock-to-lock. This is one way to tell which one you have, but very often a rebuild includes the quicker steering of the later models anyway. So either way you would have the better setup.
The rag joint I used to connect steering rods is brand new.
Rag joints are typically fairly tight. Even though they're intended to be "flexible" in a way, they have good rigidity radially, so don't often contribute to play when new and installed properly.
Of course, as you can see from DuneBuster's reply below, that ain't always so...
I've lifted it it up with a jack already, and nothing seems loose.
You can't use this test. It's pretty much useless until things are really worn. And with most of your stuff being new or almost new, you won't see much using your hands. What 69broncoCT said is the way to go. You need a helper, but it can be a real eye-opener sometimes.
The vehicle does have a 2.5" lift, but I added a drop pitman arm. I realize it only requires the drop for 3.5" or more lift, but I needed an arm without the key holes so that I could keep it on center.
Good for keeping it on center. You can also file out the master splines to let your stock arm rotate out of phase. But that's not working as you think. If you need to rotate the arm on the box, then the box in NOT centered. The correct way to correct this is with an adjustable draglink.
You need to do two things here then. You absolutely must install a dropped trackbar bracket to match the dropped pitman arm or you're just fighting yourself. You must also re-center the steering box. Both of these things are fighting your good steering and I'm surprised you only notice it at 60 mph our more.
You need to fix those two things one way or another before (or at least at the same time) you do other things.
The only thing now is learning about suspension and steering, because that is the only thing left to do on it. (until I fix it and another problem occurs)
Haha! Yep, that's the way of the Broncos. Fix one thing then some more drama crops up somewhere down the line.
But yours is at least now coming clear. Your steering system on these trucks is best when both the draglink and the trackbar are parallel, the same length, and pivoting in the same plane.
Just as important is keeping the overall angles as low as possible, which is why I personally think it's a good idea to use the drop arm and bracket even with just a 2.5" lift. It's not "necessary" but it does help.
Your box has it's on-center position which is the only place it should be when the truck is traveling straight down the road. This is not quite as important as the other aspects, but it's still quite important.
Double check your center with the draglink (or pitman arm) disconnected so you can put the box to both limits and count the turns precisely back to center. Then make sure your steering wheel is centered on the box at this position so you can use it for reference from here on out.
Then keeping your steering wheel straight when going straight means you know your box is centered. The draglink (like this:
http://www.wildhorses4x4.com/product/Adjustable_Drag_Link_30in_6675yr) will let you get there.
The trackbar drop bracket also serves to get your axle closer to centered with the lift. An adjustable trackbar is your friend here, letting you fine-tune the positioning, but with the drop and a 2.5" lift it's not completely necessary. The draglink is still a good thing though.
Good luck!
Paul