I'd check the simple things first.

Make sure the lights have a good ground. There should be a ground wire somewhere (either off the battery terminal or from the engine to the body). Many trucks also have a small ground wire from bed to the cab or body... make sure it's connected. You can run a temp ground wire from the frame ground wire on the lights to do a quick check. .

Next would be the plug on the wiring harness that comes out of the steering column. Besides the plug being tight make sure none of the individual wires have backed out of the plug.

If the problem is not there I suspect yo will find you have a bad turnsignal switch.