First problem -- where are you going to find the hubs to mount 19.5 inch rims?
Second problem -- doesn't make much sense. The tires have way too much load capacity for your pickup and the ride will be real harsh. I use load range e tires (because of "tire / size availability" in my preferred brand) on my t/a and the ride is harsher than a load range d tire which is more than adequate for the gvw of the t/a.
Third problem -- tires are designed for their load capacity. Airing down the tires to get a "softer ride" (May not be possible) will result in flex. Flex = heat. Heat destroys tires.
Fourth problem -- weight of the rim / tire. The extra weight will cause more stress / wear on axle / suspension components - like axle bearings, wheel bearings, tie rod ends.
Because of the limited availability of tires for the 16.5 inch rims that are most likely on your pickup. You might consider changinfg to 16 inch rims to get a better tire selection.
As far as tire mileage -- I have replaced the michelins I have used more because of age than the "tread was gone".
The current michelin ltx m/s 245/75 16 tires on my t/a have 11/32 of tread left (out of 15/32) (a quick check) -- the tires have 38,000 miles on them (tires have a "hard" tread).