That would be negative camber ...

Gasser or Diesel ??

I'd bet springs ... If it's a Diesel I'd say for sure! You said it gradually gained negative camber ... Sagging spring clue.

Measure the distance between bump stop and spring, that'll tell you your spring sag.
What it should be I have no idea.

If the springs are sagged you may not be able to get a bushing with enough offset to bring the camber into spec.

Sounds like the leveling Add a Leaf kit is exactly what you need.

Wow, You take good care of your farm truck ... Most farm trucks I know get no new parts, they get hacked, welded or whatever makes it go again with little care to how well as long as it goes!

And to pay some one to do an alignment on a farm truck ... I commend you on taking care of your equipment! Thats awesome.

-Enjoy
fh : )_~