It just hit me that my trucks (`05 Silverado 1500 with tow package) hitch may not be rated for the boat that I pull. Been towing my boat, about 6-6,500 lbs including trailer for the last 3 years, without any problems , with the factory Class III, 5k rated, hitch that came with the truck. So far I have had no issues but should I immediately upgrade the hitch to a Class IV? What is the basic difference between the two? What is a good brand and/or vendor?