When checking glow plugs with a test light, I should unplug the wiring harness at the valve cover and and test them there. Question is which wires are for the GP's. There are 8 wires, are they the 4 that are toward the rear of the engine or the 4 toward the front of the engine. Truck has been starting great all winter and in one day nothing without plugging it in. Doesnt sound like the GP's to me but was going to check them anyway. Batteries look like the stock one's when new but not sure how to tell (they are motorcraft and look pretty old) truck is a 99 CC 4wd auto swb

 

On your truck, I believe the glow plugs are the two outboard wires at each end of the harness, but before you take that to the bank, someone with a Superduty would have to verify that.
I'm an OBS guy.