The speed the flasher works is based on current draw (number of bulbs). So you have a burn out bulb or corroded socket, bad wire, etc. Just do a walk around with the turn signals or 4 ways on. You already know what side, now you must find out front or rear.
Sylvania web site lists every bulb by vehicle. I would print it out for your truck and put it in the glove box. I have one for every car/truck I own.