Need help with a 2015 Freightliner dd15 harmonic vibration

Sounds like its engine/transmission related then if that's the case.

 

You can rule out drive shafts and rears then. I'd say it's in the trans or engine. Does the engine feel or sound as if there is a miss to it?

 

Injectors are what I would be looking at then.

 

I would start with the basics. Make sure all your engine mounts are tight, engine is mounted solid to the transmission (ie bellhousing bolts aren't loose), pop off the inspection cover and check for missing or broken parts. Really anything that could be related to engine mounting or engine balance.

 

Now I just noticed you said something about a miss. Yes I would consider injectors as well in that case.

 

It might not be an injector, could also be a wire or wire harness issue. Had a friend that had an 09/10 Cascadia, and had troubles with broken wires or corrosion in the engine harness. FL dealer that found it said it was a fairly common problem with them. I don't remember what symptoms his truck had though.