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 totally restart the game. If you already have four of them completed, just joining the Rockstar Social Club and linking your Gamertag will unlock your hydraulics and air bags.