Are you sure you have them wired up correctly? It sounds like the wiring is connected backwards. It may be wired incorrectly at the grille harness, from where it connects to the bulkhead connector next to the radiator, or the wires may be mixed up and running to the wrong bulb connectors.
The way these four lamps systems work - High beam bulbs only have one filament and a two prong connector on the back. The low beam bulbs have two filaments and a 3 prong connector in the back. The two filaments in a low beam lamp are 1. High Power - this is the normal low beam lamp output. 2. Marking light/dim power. When high beams are selected at the dimmer switch, the normal low beam filaments in the low beam lamps go out, and the dim filament lights up to serve as a marker to show you have 4 headlamps.
You may find the wiring diagram in the following thread helpful in debugging the system. Just ignore the relays and follow the wiring from the battery and dimmer switch to the lamps. You'll notice the high beam lamps are wired to the low power filament in the low beam lamps. Something in this circuit is the source of your problem:
http://67-72chevytrucks.com/vboard/s...d.php?t=387693