What would cause a engine to backfire really loud then stop running?

Sounds like an ignition problem. You need to verify if you have spark.

Check the basics first, see the engine is mechanically OK, check compression is even in all cylinders, check cam to crankshaft synchronization if it is suspect. Check ignition timing and fuel delivery. To check compression the spark plugs will be out look that all plugs look the same and uniform in color, make sure no plug shows signs of mechanical damage or oil fouling.There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online