so my steering wheel vibrates at about 80 mph.
should i re-balance only front 2 wheels or all 4?

Am more then sure you have a bent wheel..When you car goes to the right or left then they need to be balanced.

I have the same problem. The dealer told me I needed to get them balanced but they didnt say if just the front 2 or all 4 and I forgot to ask :/.

i just got all 4 balanced and no more vibration

no wheels bent.

You're kidding right? Balance doesn't cause your car to track, the caster in your alignment does.

If you have high speed vibrations you need to have the assembly balanced. If you have aftermarket wheels and don't have hub-rings, that can be another culprit.
-GP-

You can do just the front 2, but you might as well do all 4 since you're already there and the car is already in the air.
-GP-

Road Force Balance? Is it real?

Is it real? haha, yes, it's very real. I performed a road force balance on a car yesterday. The tire was marginal to start with and had a pretty bad hop with 24lbs of road force. After the ride matching it had no hop and only 12lbs of road force. It can also detect if a tire is defective.
-GP-

The dealer charge me once like $70 for a Road Force Balance a while back. It was the only period of time when I could drive with my stock 17" without vibration!