If the engine is running rough with a popping sound it could be a head gasket. There is a fuel/water separator that could need service, it should be on the firewall behind the head on the right side of the vehicle. Unburned fuel will come out the exhaust white, the thermostat could be stuck open not allowing the engine to reach operating temperature. Another possibility is a cracked compression ring, there are more on diesel pistons to help achieve the higher compression ratio required to ignite diesel fuel. A compression check will tell a lot of this story but aren't much fun to do on the 6.5 turbo's; a compression tester capable of 400 psi is required with the adapter to thread into the glow plug holes then rotate with the starter until the needle stops increasing, usually about 3-4 good jumps then it will level out and the tester will retain it until released. A good way to find out where to start is start the engine when it's cold and feel exhaust runners on the manifolds and start with the one that seems coolest then the ones on either side. The 94 would be the first year with electronic fuel metering which had a flaw built into the PDM which would usually manifest itself by hiccuping while driving then fall flat on it's face and restart while rolling or if sitting still it would usually start right back up leaving you wondering why and pointing fingers at a clogging fuel filter. Start ups would get harder having to cycle the glow plugs multiple times to start followed by excessive white smoke from being over fueled.
If your not up to those checks yourself go to a GM dealer, Bosch-Stanadyne stood behind the failing PMD's very well and the warranty extends out pretty far mileage and year wise.
I hope this helps and feel free to let me know what you run into and I might even be willing and able to help you out of the disabled truck if you decide to give up on it.
One more afterthought could be a stuck turbo wastegate pumping too much cold air into the intake to allow the fuel to ignite.