Was there a period for which you were unemployed or earned substantially less than normal? That would explain getting behind. As for what to do about it, check the laws in your state. Where I live, if someone gets behind and is a W-2 employee, they simply start taking more money out until the back support is paid up. Each state has a different maximum percentage they can take. If you're earning a lot less than when the child support was set, then you may be able to get it adjusted. Take it from me, the amendment process through the courts is horribly tedious, but if it's been more than three years and any changes will constitute more than a 20% change in your support obligation (actual percentages may differ in other places) then if the laws are the same there as they are here you should be able to request a review through the child support office to determine whether or not you're still being expected to pay a fair amount for your circumstances.
Luckily, while they do send letters threatening contempt of court and all that, if you demonstrate that you're trying to get caught up then they are often willing to work with you. I don't know how far behind one has to get before being in danger of losing driver's license and passport, but it seems to be pretty far into the game. I'm currently very behind on my own support obligations because of severely limited work throughout a pregnancy, then my husband became unemployed several months ago and I barely make enough to cover essential bills, much less be able to send child support. So far no additional action has been taken other than informing me I'm behind, but I've also contacted the caseworker to let her know the situation and she gave me some great advice on how to get it straightened out, and has been patient so far while I try to act on her advice. My situation is slightly different than most because I'm self-employed, they can't do a wage deduction, but everything else is pretty much the same other than location.