I have a 1987 D-150, and it has some steering issues:
When making a sharp turn both wheels look like they are laying over to one side 15 degrees and they rub on the stabilizer bar, binding and making it nearly impossible to turn.
On the highway, it is incredibly unstable, tending to wander at the slightest deviation, and then massively overcorrecting.
I replaced a leaking power steering box recently, and everything is tight.
Could I have installed the box improperly or gotten the wrong box?
Or is it completely unrelated?
It *appears* to be a severe castor (and camber?) issue, but I'm afraid to drive it to a shop to get it checked out, as the closest one is over 25 miles away.
Any advice on what to look for?

 

I'd check the wheel alignment. Then check the bearings, the rubber parts for wear and cracking. and Look for something that is broken but not obvious. That problem sounds really bad. Here's some vids on bearings and steering linkage. Youtube has some good stuff if you spedn the time sorting through the junk.
The end of this vid at about 9:10 talks about castor setting which may apply to your problem.

 

I replaced the tires, gearbox, rag joint, tie rod, tie rod ends, drag link, all four shocks and installed a steering damper. (Can't remember if I did ball joints or not.)
It is MUCH better now, although there is still some play in the steering wheel, making for constant tiny corrections on the highway.
Since I will be towing a car several times this summer, I would REALLY like it a bit tighter. This truck only has 68,000 actual miles on it, and I'm planning on keeping it the rest of my life- I just don't want it to *shorten* it.

Adjusting the sector shaft on the box tightens up some of the play, but it got to the point where returning to center was iffy.

Is it possible to install the center link backwards?

 

VICTORY!
Center link was installed backwards. The offsets where the tie rods mount should be to the rear, not the front.
When I took it in for alignment, the technician told me it was sitting too low, and I needed coil springs. Replaced the springs and got it aligned, but he never noticed the center link in backwards. Didn't say anything about the turning problem after his "test drive".
FYI: this causes the tie rods to be about two inches higher and five inches forward of their correct position. The effect is when turning the wheel more than about half a revolution, the camber and toe on the outboard wheel go straight to Hell, dragging the tire screeching across the pavement, and making sharp turns impossible.