I cruise at 70-80mph+ on the curvy highways around here with no instability at all......even when running the stock tires. In fact, I would go as far to say that a 4rth gen 4Runner is THE most stable body-on-frame "trucky" type vehicle I've ever driven on the highway.
Been up over 100mph here and there, which obviously isn't any SUV's mission in life, yet the thing was still rock solid. Hell I was cruising at about 80mph on some 2-lane once and had to mash the brakes to avoid a deer/Dodge Caravan all at once and it felt very controlled through that!
So basically......If you're scared to switch lanes in there thing, there's either something wrong with it, or you need driving lessons.