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I think you mean 18-inch wheels, not tires. On stock vehicles, large-diameter wheels like that have virtually the same overall tire diameter as you get with (what used to be) standard wheels. There's this silly trend nowadays to put very low-profile tires on pickups (it's silly because they are useless off-pavement), but that's the reason for using such big wheels. It's to make up the loss in sidewall height of that style of tire. So, unless you've modified your truck yourself (or unless the previous owner did), it won't be any higher off the ground with 18-inch rims than it would be with 15-inch rims, or certainly no more than it would be with 16-inchers.
Post your actual tire size and I can tell you exactly how the diameter compares to the old standard for half-ton trucks, 235/75-15, or to some of the other more-recent, taller options that were common before the low-profile performance street tires "took over" the market.
Was on my 93 T-100 now it's on my 06 Tundra and it will move to my next truck too.
Solid and I can move it forward to put large items in the bed.
I also have the bed cross bars so I can mount a car top carrier at the top of the bed and still throw some stuff underneath it.
I've had 4 kayaks on it and I wasn't worried about them.
As far as trucks getting too high you can always put the kayak across the bed then step into the bed and lift it onto the racks. This only works with one kayak though :-)