Need help ASAP PLEASE!!!!!!!

[bigred90gt]

I think I have a big problem. I was on the way to work thismorning, and all of a sudden I started hearing a pretty loud ticking, it was almost like an exhaust leak. The truck started riding pretty rough. When I exited the freeway and stopped at the red light, the truck would shake with every tick, and I could feel it in the stick. The stick was going back and forth about 1/2" with every tick. I got out a work and opened the hood, and I am pretty sure that it is not an exhaust leak.
BTW - The truck is a 93 F-150 4.9l I6. Any suggestions would be great. I'm hoping it is not a valve or a lifter or something. If it were a valve, would it still crank and run, or would you loose compression and not get anywhere?

Thanks for the help. This is my daily driver and I cannot afford for it to be down for more than a day or two.
Caymon