I would get it into a shop or at least a place that has a code reader and find out what codes are being thrown. Many auto parts stores will read them for free. A scanner can be had for $80 US if you can not find someone to read them for freee. It's a good tool to have.
Once you have the codes you can go from there. Without them anyone would be simply guessing what is causing the light to come on. I wouldn't continue to drive it though without finding out, you could be damagin your catalytic converters which are very expensive.
1978 F250 Lariat 4x2
2008 Expedition Eddie Bauer 4x2