1989 D100, power steering wanders a bit on the highway. It doesn't seem to have much play, but it just doesn't seem to want to recenter itself. Will a rebuilt or new steering gear box improve things?

Tires make a huge difference. Toe out too far will make it "hunt".

I'd start with an alignment. A bit more positive caster would probably help.

Robert

Check for a sloppy steering box and bad components.
Check tire condition.
Do an alignment.

Good advice here.
Over sized wheel and tire combos will wear things out quickly.
Shot gunning parts is always a very expensive and frustrating way to trouble-shoot.
My 1st thought was alignment. Might be worn parts or tires but you won't know till it gets checked out.
Our local Fire Stone store does very reasonable inspection and repair.

I agree with what everyone has told you. Perfect advice! You know how old your tires are and their condition. Even with plenty of tread if they are old they could be the culprit. Alignment as mentioned, tie rods, and I'd even throw shocks in the mix. As mentioned check everything instead of throwing parts at it.

I took the truck in for the state safety inspection and one of the failure discrepancies was "play in the steering." I have a really good alignment guy so hopefully he can figure it out. Over the phone he suggested it could be a number of things including:
- someone tightened the gear box adjustment to reduce play when centered, which makes it excessively tight when it is off-center
- ball joints and tie rod ends are stiff from lack of regular driving

I'll update after I get it back from the alignment shop. FYI, the tires are only a few years old.

If the truck was bounced by inspector, it must be fairly bad. Should be easy to spot the excess with a helper turning wheel back and forth 1/8 a turn or so...If there is a steering coupler by the gear box, that would be first place suspect. The steering box is at it's tightest, adjustment wise, when in center not turned to one direction by the way. According to shop manual. IIRC

I have trouble spotting a helper

Apparently the steering box was overtightened, so it didn't let the caster return it to center but instead would stay wherever it was put. My steering guy loosened it, so now there is actual play. He wants to let the gears wear in a little bit before tightening it back to where it should be.