When the truck is not running the headlights work great, all voltages are at 12v. When the truck is running both high/low beams work fine except for the low beams on the passenger side, they drop to 8v. Any ideas??

 

check the ground to the body at the headlight socket.

 

Update:

The low beam light will go off right after the key is realeased after the truck is started, if if i turn the key to start the truck while its already started it will come on.

 

Grounds all checked out. Its only receiving 8v to the socket when the truck is running compared to 12v when its off. Where could it be loosing 4 volts, could it be in the starter since it will work if the starter is activated when the truck is running?

 

is the truck charging? you should have approx 14v at the battery when it is running. if it is not charging, you might go down to 8v to the lights.