If your truck has a GVWR over 10000 lbs, driving for multible carriers would require having your own authority. Under 10000? There are carriers that allow you to haul for more than one carrier. You will be required to have your own insurance as you would with your own authority.
There are a few carriers that will broker a load for you if you are signed on with them
This is a much discussed subject with hundreds of posts. Try a few searches. Here's one
http://www.expeditersonline.com/forum/search.php?searchid=2101393
If you get a load from a independent load board, you, or the broker posting the load, must have carrier authority (not just broker authority). If neither of you have carrier authority you would be hauling the load illegally unless you are signed on with a carrier that will broker the load for you. There also needs to be a contract in place.
I might add, there are not many expedite loads on the load boards you will be eligable to join. You most likely would be hauling LTL at a very reduced rate. Unless you are really in a bad area, it may be better to just wait it out or do some deadheading.