Where does it sound like the "grinding" is coming from.......trans. area, diff. area, xfer case area, etc.? Just a few questions to get a little more info:
---Have you checked the fluid level and fluid condition of the transmission, xfer case and diffs.?
---Does it happen when you shift from reverse to drive?
---Does it do it just on the gear transition, or when its moving in reverse too?
---How does the transmission upshift/downshift?
---Any roughness, slipping, hesitations from the trans.?
---When is the last time the transmission, xfer case and diffs. have been serviced?
I can't imagine it being a pinion bearing grinding. If it were a bearing I would think it would be more a growl or continuous noise both going forward and in reverse, not just when shifting from drive to reverse. Not having heard it, there are a number of things that could be causing it......driveshaft(s), transfer case, transmission, front diff. bushings, either or both diffs., just to name a few.
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