To unlock hydraulics and air bags, you must join the Rockstar Social Club, link to
your Gamertag, then complete four of the twelve "Driving Test" achievements listed
on the website. Doing so will unlock hydraulics/air bags for your Gamertag, even if
you completely restart the game. If you already have four of the achievements completed,
just joining the Rockstar Social Club and linking your Gamertag will unlock the hydraulics
and air bags.