Being as the fuel cap and owner’s manual specificity mention NOT to use less than 87, that gives me pause.
It will be fine, genuinely. At altitude, air is thinner, which means less fuel is required in order to maintain a healthy Air/Fuel Ratio (AFR). For turbocharged or supercharged engines, a lower octane can be more problematic since 13psi is 13psi no matter what altitude you're at, but for our NA 2UZ engines, 85 won't hurt anything. This thinner air (and thus, less fuel) is also why you'll feel the truck get noticeably more sluggish the higher you go. Less fuel means less power.
Maybe I'm wrong, but this is how I understand the situation, as an owner of both a 100-series and a 1994 Audi S4 with a turbocharged 5-cylinder that would almost certainly leave me stranded if I put 85 in it. They're very different beasts. I also live at ~5k feet, and frequently drive up to 10k+ feet, for whatever that's worth.
EDIT: Just a thought, but if you don't already have a ScanGauge I'd recommend picking one up. Knowing your exact water temp and trans fluid temp is great peace of mind, and anecdotally, I've seen my temps creep up quite a bit while the stock gauge cluster gave no indication whatsoever (210 WT, 165 TFT). I'd rather know what's happening
before the idiot light comes on for something, and since you'll have the truck loaded down with people and gear, this will almost certainly be a concern for you.
