Grinding noise when starting in 1st, 5spd

98 4.0L, NP231, AX-15, 130k miles, etc.

When I start from a stop in first gear, I get a grinding noise for a second or two. It's only when starting in first gear from a stop. It sounds like it's coming from almost directly under the Jeep, but I'm not certain. I sifted through about 5 pages of search results and came up with nothing. Any help would be appreciated.

First try to rule out the trans or first gear.
Start in second, any sound?
Start in reverse, any sound?
Park on a hill and let it coast, any sound?
In two wheel drive, lift the rear on jack stands, block the front wheels, start in first, any sound?

Good luck!