Interestingly v8 4th gen 4runners are rated at towing 7000lbs. V6 4th gens and 5th gen 4runners are rated at 5000lbs. Newer 4runners are very similar in size to a 80. V8 4th gens and v6 4th gens have the same size brakes. So I get the feeling the power plant is a big factor. 5th gen 4runners have bigger brakes afaik at least up front. Bigger brakes can be put on a 80 series too.