I have a 91 7.3 idi and over the last month it's gradually gotten harder to start. Now, as of last night the wait to start light just flashes and shuts off. It's cranking fine but won't turn over, and it is releasing white smoke. I don't think any of the glow plugs are even turning on. Do I have a bad relay or glow plug controller?

 

The first thing to check is the glow plugs them selves. The glow plug controller senses the resistance to ground through the glow plugs (which varies with heat) to determine how long to energize the plugs. With a couple burned out plugs, the controller thinks the engine is warm, and doesn't turn them on for long (the short wait to start light...)
This thread will tell you all about the system:
https://www.ford-trucks.com/forums/9...le-system.html

 

I looked at the forum and did what they said with the test light on checking the plugs and the plugs all made the light light up which according to the forum says they're good. Then would that mean its a bad controller?

 

I'd check the wiring from the relay to the glow plugs next. If that's bad, it can have the same symptoms... If that's good, then check the relay itself, and if the plugs, wiring, and relay are all good, then the controller is all that's left...