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I would not drop to a load range D tire for a few reasons.
First, the late model SD chassis, especially CC w/8 foot beds seem to be especially sensitive to tire inflation oddities. (See death wobble and bouncing threads). Dropping to a D range means the tires cannot be inflated to the PSI Ford recommended.
Second, you are not towing or hauling anything now, but you might one year from now.
Third (and this is just my opinion) there are to things you should not be frugal with on your vehicle, brakes and tires.
Dropping to load range D tires should not be detrimental to your vehicle. The Excursions came from factory with Load range D and wre built on the super duty chassis. You should get a better ride with enough tire capacity to carry light loads.
I run LT285/75R/16 in a load range D tire. I have put over 140k miles on my truck with this type of tire. They actually have a higher load rating than the original tire. The ride is slightly smoother but the do tend to bulge or squat more if you load them to near max capacity. Of course the capacity of the tires exceeds the capicty of the truck.
I purchased Cooper SST 35x12.50x18 for my 2005 F350 and I have noticed a big difference. The stock tires were junk and I got rid of them after 15000 miles. I Would not give then to anyone!