I believe you are "required" to use a WD hitch for anything above 5000lbs.
As the others have said, check the yellow sticker in the drivers door pillar for the payload of that truck. That is what's left of the GVWR after the base truck weight plus all the options on that particular truck are subtracted from it. every truck is different depending on packages and options. The payload is figured to include one 150 lb driver and a full tank of fuel. Everything else you add to the cab, bed, or hitch has to be subtracted to get your remaining payload.
After doing the math, you'll see how close you are to the payload, and why so many get the max tow package at the least, and some get the HD payload package too.