Did you install ball joints? Did they already come pressed in? What knuckles?
I would attempt to disconnect the drag link from the pitman arm, then put the axle on jackstands, and manually turn your front wheels/tires by hand. See if you can manually feel the binding anywhere. That will at least eliminate the drag link and steering box. Also with the draglink off, you could turn the steering wheel one time each direction and that would eliminate the gear box, but don't go too far and make sure you get it EXACTLY back to center.
Also, did you already have 37s or is this a new thing too?
EDIT: You said left turn. Check that nothing is catching on your diff cover, pretty common actually. Mine was binding the first couple of times until it wore off the metal on the cover:
http://wayalife.com/showthread.php?42612-Stock-tie-rod-stabilizer-bracket-hitting-ProRock-cover