Sway bar thickness handling differences...

[RichardJ]

>>Firestone Trans-Force tires...................16.5" wheel size tires my truck takes. It has an uncomfortable amount of oversteer with the Firestone tires, even with 10 psi more in the rear tires.<<

Unless you were starting with under inflated tires, the extra pressure WOULD increase oversteer.
I know several that use the Trans-Force truck tire and I have them on my class-C motorhome. It's a high mileage highway truck tire.

If you noticed oversteer with the new tire, you should have dropped the pressure. I never go even close to the 80#, unless I expect to be hauling something really heavy.