Fireflipper
You will have to check your manual to see what viscosity oil to use in your truck. I know that most manufacturers are now recomending synthetic. Since you are probably towing a fiver I would recommend using Amsoil severe gear lube. It will be more expensive than the standard oils but it will stand up to the harder work that we put on the rear end while towing. You can order online or find a distributor near where you live.