Suspension 88 GT Steering Wander At Speed - Zero static steering play

Did alignments for a living 20+ years at Ford
Replace the rack and pinion if the toe set is correct +1/32 and live with the outside edge wear
The sector shaft is most likely sticky
You can try messing with the adjustment
The spring loaded plastic piece that puts pressure on the rack is supposed to be lubed with polyethylene grease
Overhauled several racks under warranty
Replaced tons of parts control valves sector shafts and racks Lincoln MK IV-VII was the worst for sticking
The only way to keep the front tires alive on a Mustang Tbird or Mark
Is to set the toe at - 1/16 or -3/32
That will make the tires wear flat but build in a wander
There are several old bulletins about the alignment specs for a Mustang T bird chassis
They all have the toe specs being adjusted in the negative
Camber -.5 degree both sides
I used to set the RH camber closer to -.75 and the LH Camber to - .25
You can throw a bus sideways thru the specs and all alignment folks will tell you yours is within spec after they are done
The Ford dealer should have an expert alignment guy
Make sure he knows you are fighting a wander complaint
He will set your toe to +1/16 or so and solve your issue

Ive seen kids clamp the rack adjuster down to force it straight down the road. Not the way to do it.
Your issue is in the camber and caster. I set mine to -.75 on both sides and pulled the caster back as far as I could. Car sticks like glue.

Just because the car isnt all miled up, doesnt mean something wierd didnt happen in the bushings. If it sat and they got hard and cracked and then you drove it, you could have some weird stuff happening. Has anyone ever put a pry bar on the bushings and checked?