My Laramie came with the 18's and I got a second set for snow tires.
Found mine on Craigs list for $400 for the set,
when I was looking for tires I was looking at 255/70r18, 265/70r18 or my stock size of 275/65r18.
I ended up with a set of 265/70r18 LT's as I found a used set with 60% tread and only 2 missing studs on all 4 tires for $400.
They are Nokian Hakkapeliittas which are one of the better if not the best snow tires, this will be the fifth vehicle I've run with this brand of snows.
I much prefer and get better results with the narrower tire, both in traction and how much less the truck pulls when hitting slush pools on the road.
I've been stalking a set of take-off 18's since I ordered the truck in April and have yet to come across any locally, unfortunately. I've searched out up to about 200 miles and the best I've found is around $750 (without TPMS) and a heck of a long drive for them.
As far as tires, I've run Nokian's on a bunch of vehicles over the years (everything from WRG2 to various gens of Hakka's). My little SUV is on them now, although I am very close to needing a new set. Obviously I have nothing against Nokian tires, but the stock on them is limited and I'm concerned about being able to find them in the appropriate size if (when?) I eventually locate wheels.
With that in mind, I just ordered a set of 20" Blizzaks because there were only 3 sets locally and no assurance more would be available when those sold out. The shop will hold them for a couple weeks for me. If I don't find wheels in the next week or two, I'll just have them put on the stock rims on my truck. It'll be far better than running all seasons, at the very least.