Oh ya, 285/75R18 is too big, and unless you're lifting the truck or adjusting the wheel spacing, will rub.
Check out the official 'largest tire you can run stock for all generations thread':
https://www.tundras.com/threads/largest-tire-you-can-run-stock-for-all-generations.9762/On the 2nd and 3rd gen trucks with stock suspension, you can run a 275/60R20, which is one inch taller but same width as factory. Let's compare a 275/60R20 to a 275/70R18.
275/60R20 has a diameter of 32.9" - one inch taller than factory
275/70R18 has a diameter of 33.1" - one inch taller than factory
I seriously doubt that .2" is going to make any difference, and you should be fine to run the 275/70R18's on your truck.
The way I see it, you have two options: Stay with the stock 18" size, which is 275/65R18, or step up one size to 275/70R18. Either way you can't really go wrong.
Some people put spacers on because they want to A) run a wider tire and don't want the tires rubbing the inner fender liner, or B) want the tire to stick out past the side of the truck. I personally find the look where the tire sticks out past the side of the truck to be quite stupid, and I'm pretty sure it's also illegal. It's not too bad if you're running some fender flares. Then again, spacers are stupid, and you're adding in a point of failure on one of the most important interfaces on your truck (wheel to axle).