You can weigh your truck to get an accurate unladen weight if you like.... or go with an estimate if you think you have a pretty good idea of what it weighs. The 6500 lb rear axle weight rating is really a rear axle spring rating. To carry a heavier trailer, adding air bags will help. Your tires are likely rated for something like 3640 lbs each, so your rear axle with air bags could handle about 7280 lbs. The unladen rear axle weight of your truck is likely around 3500 lbs. So, if it were mine, I'd add air bags and be able to carry a 3500 lb pin weight without being concerned.
