Hearing protection until they fix engine fan

It's probably a loose connector or air fitting somewhere and probably a relatively easy fix once you find the problem causing the fan clutch to remain stuck in the engaged position. Shut off the engine, then turn the key to the on position (don't start), then carefully listen for leaking air under the hood, or fiddle with connectors and harnesses and see if you can hear the clutch disengage.

Is the AC button off? (maybe the AC or defroster system is faulty and calling for fan to be on) This can sometimes be caused by having a refrigerant leak or otherwise too high pressure in the refrigerant system

 

I'd bet that has more to do with the noise and affect on your hearing than the fan. Most class 8 diesels don't like to be spun up that fast. Normal operating range is only going to be 1500-1800 at most, but probably even lower than that these days. Instead of trying blow up the motor turning 1900 rpm's, grab the next gear. If the next gear won't keep you moving, ease out of it and drive a little slower to keep it within the operating range of the engine. It may not even be a problem with the fan...the computer might be telling the fan to engage because you're running it too hard. Ease out of it a bit and see if that helps...try to keep it at 1500-1600 cruising RPM. Whatever speed you can maintain in a gear that puts your RPMs in that range is where you'll have to be satisfied with running.

 

All I can think of is get the truck fixed. Probably not the best idea to be wearing ear protection while driving, IMO. As I have flashbacks to screaming back and forth to my dad in the passenger seat in the old GMC Brigadier 6-71 Detroit with no muffler. LOL Have you tried cotton?