If the trailer has electric brakes, I wouldn't be too worried about it. I recall hearing that GCWR is based more on engine horsepower than on braking capabilities, because the people who came up with the number assumed that the trailer has brakes that are capable of bring it to a complete stop...and looking at a owner's manual confirms that; the GCWR's based on the power train, not the truck weight rating (i.e. braking capability). The owner's manual also states that most trailers weighing over 1500 lbs are required to have a braking system, FWIW. (I admit this was news to me).
The owner's manual says that your GCWR's 12K if you have 3.55's, or 14K if you have 4.10's, or 17K if you have 5.13's. BTW, those numbers are 11.5K, 16K, and 26K for 460 trucks
With that said, you might want to check the laws in your state...I know that, in CA, if you're towing a trailer that weighs over 10K lbs, you need a Class A CDL...
As to legality...as long as the trailer brakes are in good working order, and as long as the tongue weight doesn't bring the truck over GVWR/GAWR and the trailer's not over its GVWR, you should be good