That truck is equipped with the 5.9L Cummins 6 cylinder diesel engine. As long as it was maintained properly, it still has some miles left in it.
I pulled the build sheet on the truck and it is attached to this reply.
As far as what that truck will tow, it all depends on total weight of the truck loaded (Gross Vehicle Weight) + the total weight of the trailer loaded (Gross Vehicle Weight), compared to the Gross Combined Vehicle Weight Rating (GCVWR) of the truck.
Your truck has a sticker on the door jamb that shows the truck's GCVWR, i.e., the totally loaded, hooked up to towed vehicle, weight rating of the truck. You'll need that number.
Here are the specs on that truck:
Gross Vehicle Weight Rating = 11,547 (weight of truck, passengers, gear, etc... trailer pin weight)
Towing Capacity = 15,400
Dry weight = 6,876
Payload = 4,671
Your truck weighs about 6876 pounds without anything in it. The truck's payload rating, i.e. things that you haul in the truck, is 4671 pounds. So, add up the weight of fuel (6.9 pounds per gallon), passengers, gear, luggage, stuff in the bed like tool boxes, etc... + add 15 - 20 percent of your trailer's weight as a pin weight number and there you have the truck's actual Gross Vehicle Weight.