Please describe this you're you calling a stabilizer? Is the drag link new too? '78 model has two ball type joints in it. OEM has the better steering linkage.

How did you get 8 degrees caster? Who measured the 8 degrees. It is positive caster I hope. If the lift is just taller springs with standard C bushings and no radius arm drop at the frame, could be negative caster which will be all over the place. When you turn the wheels hard right or left do they look like the lean forwards or back?

Is you track bar and the drag link both roughly parallel to each other? If not they'll swing through different arcs giving you bump steer out the wazoo. Is there a dropped pitman arm and a track bar relocation frame bracket? Truck looks high to me.

I have a stock height '77 f-150 4x4 that I've had since 1986, 31/10.50-15 tires, '78-'79 type steering, Red Head steering box, new track bar and tight tie rod ends, red polyurethane 4.5 or maybe 6 degree C bushings truck drives like an arrow. When the wheels are turned left or right, they lean back. I probably have better than 7 degrees positive caster.

 

He said he has James Duff long travel radius arms. They come with 4.25* caster built in. The only way he has 8+ is with a set of 4* C-bushings.

I have the same arms on my 78 Bronco, but with more lift than he has (6-7") and I have around 8* caster (4.25 from the arms + 4* C-bushings). Mine doesn't pull to the side when going over manhole covers or pot holes, but I also put on other items to compliment the lift springs (track bar drop bracket, drop pitman arm, adjustable track bar, etc).

It sounds like either a slight geometry issue or you still have some worn out or seizing parts. Even with a worn out steering gear, your truck should track straight. The steering gear is only going to affect how sloppy the steering is when you're steering. If you're just going straight and hit a pot hole, the steering gear isn't jerking you off to the side. Before I did my lift and before I replaced my steering gear with a Redhead, my Bronco would track straight over holes like that, but my wheel needed to be turned like 1/3 of a turn before the wheels would move. That and it had a loose pitman arm and a cracked sector shaft. It's come a long way since then.

 

4 degree bushings with 4degress cast into the head unit of my James duff extended radius arm kit. That 8* is the theoretical however I can visibility see positive caster when then wheels are locked side to side. The drag link is close to parallel to the trac bar. The stabilizer I’m talking about is the shock/damper on the drag link to the crossmember on the frame. I don’t have a drop bracket or drop pitman

I understand the axle will swing from driver to passenger side as the suspension moves up and down but with my trac bar and drag link parallel that shouldn’t cause bump steer right?

And yes the drag link is new from LMC I believe

 

Otherwise , all I can think of is either tires just are squirmy in the tread or maybe wear in the steering box, outside chance is a frame crack at the track bar bracket / steering box but that's not likely in my thoughts ... unless hit in a prior life. My '77 only has 106K miles but for years I wanted a Red Head steering box as I had play in mine, I had mechanics tell me needed adjusting but I know they have more play straight ahead than turned and if not adjusted right, can accelerate wear as they get tightened in turning from straight. Not selling Red Head boxes, but they re-machine the places the ***** run and use new ***** and they add a needle bearing on the sector where stock just has a shaft through a hole, etc. They also build up and re-machine needed places, they have a great rep. I like mine, but was over $300 years ago.

Another thought, just a thought ... is rear suspension that allows excessive side to side can have a driver chasing it through the steering, things like bushings, broken leaves, loose u-bolts, etc.