I see this is your first post, so welcome to the fun!
Now, for your problem. Diesels are simple. They have no valves to control air flow which means if the air filter isn't clogged and the exhaust doesn't have a fat rat living in it. Air is normally not the problem.
That just leaves fuel and heat. Since you can get yours started. The glow plugs are working. You didn't mention any loud banging or knocking or smoke. Which to me means the engine is probably mechanically sound inside.
Fuel delivery is what you have left. Not knowing your truck I will list out problems experienced by other CUCV owners:
Fuel pick up sock in the tank- It could be clogged or the tank could have enough algae in it to not allow more than just a trickle of fuel to get sucked up.
Rubber fuel hose under the passenger seat- There is a rubber hose between two sections of metal fuel line. If it is cracked or has a hole in it. The lift pump will suck in air and you won't get any suction at the tank to pick up fuel. Air molecules are smaller than fuel molecules, so a small hole will not show any signs of fuel leakage.
Lift pump- They look just like a normal Chevy gasoline engine fuel pump and they pretty much are. Work the same too. If you aren't getting at least 3 psi output, it isn't doing the job correctly. More hoses to check for air and fuel leaks there too.
Square fuel filter and base- This is probably the number one source of fuel issues on these trucks. The square filter and base will crack or seperate allowing air into the system. Or, one of the sensors on the base will cause a leak of either air or fuel. Is the filter new and clean?
Injection pump-This does the job of a carbuerator, distributor, points, rotor, coil, cap, and plug wires of a gasoline engine. Except it has to do it with much, much more presicion and while boosting the fuel psi. They have an accepted shelf life of around 5-10 years. There sure are a lot of them 25 years old running around right now too though.
Fuel return check valve and lines- Any fuel the IP doesn't use or fuel the injectors don't shoot go back to the tank. There is a hose on the very top of the IP that connects into the return lines from each cylinder bank and then goes back to the tank. Pull the line off the IP and then use a 9/16" socket to pull the brass check valve. Look inside. There should be a glass spring loaded ball in there. Clean it with some B-12. If you get a bunch of junk out, you have a dying IP.
There is also IP timing and injectors to look at. However, timing can't be done except for a close guess unless you have some special tools. Injectors are easy, though messy. Fire it up and use a 3/4" open end wrench to loosen each line one at a time at the injectors. You should get a spray of fuel and a decrease in engine idle when you do that. If all 8 cause the same reaction, then start saving money for a new IP. If one or more don't cause an idle change, pull those injectors and replace them.
The military went to great lengths to make up manuals for these trucks. The -20 manual for the CUCV's has a great step by step trouble shooting section for checking this stuff out. I didn't read it before I wrote this. Mainly because I wanted to give an overview instead of anything real specific. You didn't give enough information to get too precise.
Things such as a bad harmonic balancer, broken or cracked flywheel, broken or cracked torque converter, bad motor mounts, bad transmission mount, cracked transmission bell housing all could also cause a vibration and everything on the engine could be perfect.
