I'm not sure what you mean by "4wd highway mode". The NP-231 is a dual range t-case with many applications.

As you have a dual t-case setup, I would look into driveline angle for your source of vibration, especially as the vibration occurs in 4wd.

Another possibility would be the vibration coming from within the T-case or the output shaft.

Use the site search at the top of the page to see several 4x4Wire article dealing with driveline angle like the link below...

http://www.4x4wire.com/tech/pinionangle/

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