The 4WD-low explains it. Remember that the T-case reduces the gearing in 4WD-low (2.73:1) such that the transmission output has to spin 2.73 times to spin the T-case output once. So that means if you any forward movement (even if it seems you're at a near stop) the trans output is going to be spinning quite a bit faster. As a result, it grinds when you put it in park.
I believe the owners manual recommends not putting the transmission in Park when in 4WD-low, it should only be done in 2WD or 4WD-high.
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