Since the DRLs are working the trouble is most likely with the dimmer control switch or the DRL Lamp Relay in the convenience center. Tapping on the relay or the switch may help check that. You should have at least a test light probe for testing so you can check for power.
With the light probe check the purple wire for power on the DRL relay with the low beam switch selected. You should have power there. If you do then check the tan wire which goes to the lights. If you don't have power there then the relay is bad. If you do have power then there is a wire splice on the wire going to the lights that should be checked.
If you plan on keeping the truck for some time I highly recommend you invest in a FACTORY service manual for it. It can help save you a lot of money on shop repairs for problems like this. Check Ebay for some good prices on one. The vehicle is getting older and you have to expect problems like this every now and then.