I suspect that the 235/75R15 tires may be the problem. They are not stock. You may be able to make them behave with more toe in.
I run 215/85R16 tires on my trucks. They are not stock either, but they are skinny like the original bias ply tires. They run very true as long as every part of the suspension is maintained and set up correctly.
When you come out of a turn, does the steering come back towards the straight ahead position on its own? If so, your caster is likely correct. Proper toe in (or additional toe in for non-stock tires) will prevent wandering.
