As stated earlier you should have a scanner most anything less would most likely be a guess.
If it were a LD truck or a car I would scan for codes then see if I could isolate it to a specific cylinder and proceed from there and read the scanner data related to the fault code and proceed from there.
Im not a HD truck guy, that I had a friend that had a HD Freightliner Moho conversion I cant remember the diesel engine size but when he pulled in and pitted next to me he was in limp mode / Low power and that thing was banging and shaking and I though the engine was coming unglued.
He dumped his trailer and the rig was then towed to the local Freight liner dealer and they diagnosed and fix was a failed fuel injector.
To this day I cant believe how poorly it ran and the noise's it made.
I know I'm of no help but just throwing it out there because it may not and probably is not as big of a disaster as you might think.
Now IIRC the cost of the diagnosis and fuel injector repair was pretty dern pricey.
Good Luck &