The PO put aftermarket wheels on it with these huge tires on it. Not sure what it did with the scrub angle.You said your tire size are on 18 inch rims?? Do you have some aftermarket wheels with a crazy backspacing that has drastically changed the scrub angle?
It's a pain in the ass and can become a little dangerous for a home wrench monkey who only has a jack and jack stands, but it may be very telling if you can raise the front of the truck up by jacking it up under the frame. This will get the front axle to hang down under it's own weight, and then a loose bushing will show itself more readily. Also, have you looked at the frame where the steering gear bolts on to it? Also look at the welded connection at the frame where the upper end of the pan hard is attached. I had cracks in both that had to be rewelded.
But what you described is exactly what I did. Initially I found the TREs and steering damper. So I changed that. Last night I saw the stabilizer bushing on the DS was worn.
I need to change the caster, but struggling with finding the right option. I have a 2'' lift. Not sure whether to just go with OME bushings (and finding a place that can push them in for me) or plates. The amount of lift, the availability, etc is what I am hunting now.