This problem is caused because the front lighting system is a positive ground. Meaning the ground of the bulbs is always at battery positive and all of the switches provide a ground path. In the conventional diagram, component grounds are hooked to battery positive.
A halogen bulb in the low beam slot will have its ground connected to the positive line, its high/low beam pins are connected to normally floating circuits, meaning they are not connected to anything when off. When you turn on the low beams, the headlight switch connects that part of the circuit to ground while the high beam circuit remains floating, this will cause current flow through the DC-DC converter (it doesn't care about polarity as it uses a FBR internally). When you hit the high beams the same thing happens on that part of the circuit. So far were good, the DC-DC converter gets it current flow and powers the lights. The problem arises with how the quad beams and fog lamp relay works. On most domestic vehicles, the fog lamps turn off when the high beams come on. Since the high beam circuit is normally floating, its at battery potential, when you turn it on, everything after the bulb drops to ground potential. This means that when the low beams are on, the fog lamp relay can draw 12v power from the high beam rail and the switch just provides the ground. This works because the relay only needs about 75mA to turn on, the high beam filament has a resistance of a little under a quarter ohm whereas the relay coil has a resistance of about 160ohm, so as far as the relay is concerned, the bulb isn't there, only the battery. The fog lamp relay sources power through the bulb, since you put in LEDs which are an active component, the high beam rail is no longer sourced power from the high beam filament as the diodes in the DC-DC converter prevent current flow through (its not that simple, but thats how it works) So, the power must be sourced from elsewhere. Also on the high beam rail is the high beam indicator and quad beam relay, both of those must activate with the high beams, since the rail is pulled to ground when active, those two components must receive power from the battery. Since the high beam indicator is a halogen bulb it can provide some current(its resistance is much higher than the halogen high beam) and the quad beam relay also has a resistance of about 160ohm. So when you turn on the fog lamps, it must draw power through the high beam indicator as well as the quad beam relay, this is enough current to dimly light the high beam indicator as well as activate the quad beam relay.
The only solution is to go back to halogen (since LEDs in the stock housings blinds anyone in front of you because the spread is higher than the truck) or put a load resistor across the high beam rail thats rated at about 25watts.
