Funny, I traded in my Scion tC for the 4Runner...but my Scion was not paid for, and like any good 'new car purchase negotiation strategy/approach', I never brought up the trade in until AFTER I negotiated the price on the 4Runner. It worked out, they paid it off, and the extra went to the 4Runner price reduction.
If I look back now, I would have considered selling it on my own simply to make a few more bucks. If you already have 'regret' on the mind...you won't get that out of your head. Keep it until the day you realize, 'I don't drive it enough to justify a 2nd vehicle' then sell it for the cash to put towards the 4Runner pay off or even upgrades/mod's.
Great work on finding a low mileage rig, you did indeed get a good buy. Tires, I bought BFG Long Trails, they were already on it, so it was like an OE replacement, and I spent $800 in all, at Discount Tire. (that's with mount/balance, et al). I am dreaming though, of BFG A/T's in the future when these wear out...