I vote for Stillen Sways and Z H-Tech's (It's the combo I have). Your $500 should cover price and install if you shop around a bit.

^^ I'd also listen to the recommendation to get new end links for the sways. My OEM's worked fine however I got this 'clicking' sound which sounds like the rear axle clicking at slow speeds.... kind of annoying but not THAT big of a deal. Turns out it was my end links. I got them replaced by OEM's but assume they'll go bad after around the same time period. I'll upgrade them then.

 

any good sites to buy these from guys???