I just towed my 1970 Mustang from L.A. to Eugene, Oregon. 850 miles up I-5 with a couple of 4000'+ passes.
U-Haul trailer plus Mustang was around 5,600 lbs., or a bit over the official 5,000-lb. towing capacity.
Even with the rear suspension supplemented with Air Lift airbags at 30 PSI, the tongue weight was enough to put the nose in the air a bit.
Handling was completely stable and secure. I cruised easily at 60-65 mph in S4 and S5, controlling the throttle myself. Even on level ground with no headwind, I couldn't use cruise control at all -- it caused too much hunting in and out of torque converter lockup. I took the steepest uphill gradients near the passes at around 50 mph at 4,300 RPM in S2. S3 or S4 provided engine braking on downhill sections.
All in all, I came away impressed with the 4Runner's towing chops. I wouldn't want to tow that much weight with it every day, but for a one-time trip it was fine.