Unless you go to 16" rims, it's almost impossible to purchase tires like the "E" range radial LT truck tires. Some time ago, there were discussions about fitting junk yard truck rims on trailers. My trailers have the same 8 bolt pattern as Ford trucks. However, the wheel stud diameters of one of my trailers is physically smaller, and the rims would be incompatible.

Right now, decent "E" range 16" LT truck tires, are selling for $170 - $235 in my area. Add in the cost of a new rim, and the dealer's price may not be completely out of line, depending on the brand of tires he will provide.

One thing you should check if you decide to install larger rims, is if the newer tires will fit under the fenders. You can reduce the overall heigth of the tires, by switching from an 85 series sidewall to a 75 series while maintaining the same widths.

If you continue to use the 15" rims and tires, keep your new ST tire for a spare.