I don't have a wiring diagram for a newer truck but if they're anything like the seventies cars,which is highly likely,then the power goes through the hazard switch first then through the turn signal switch then to the lights.
If the front and dashboard turn signal lights are flashing then the hazard switch and flasher are good and the problem is most likely in the turn signal switch.The turn signal switch has a lot of different jobs and are known to fail quite regularly,luckily they're pretty standardized and new aftermarket ones are pretty reasonable.
If you have a wiring diagram for your truck then you should be able to find the correct wires to use a test light on at the plug coming from the steering column.
