I would also look into your tires/rims/ujoints (and idler arm) as well. Since your vibration is only intermittant, than I would consider suspension/steering last when diagnosing the problem. Make sure your steering doesn't have excessive play in it (sure you already did that though).

How big tires are you running? I have 33x12.5's and sometimes I'll just get small vibrations because of the bad highways in RI. You're SURE all 4 are balanced? U joints are snug and unworn? Is the vibration only limited to front end? Any other driving noise elsewhere?