but our trailer is not real big, only 16', and it's loaded gross weight is 3500 lbs. Our 2001 4Runner's towing capacity is 5000 lbs, so we're about at the limit with our little Bambi + the other goodies that come along. We're not the fastest up the steep mountains but we get there, and, knock wood, we've never had a problem. We use heavy duty brake pads, so braking is no sweat, especially if you're downshifting to help slow down the rig. We've got a transmission cooler too. So far so good. Regular maintenance is a must!
All that being said, I wouldn't consider towing anything bigger or heavier with the 4Runner. It's just not cut out for heavy duty towing.