dont know about grinding
but have you tried running with the front prop disconnected?
the front output shaft bearing in my Samurai t-case went and made funny noises in 2wd, but not in 4wd because of the tension on the prop when it is engaged. i've had it do the same thing with broken UJs as well but grinding doesnt sound good.
may be worth taking the front prop off and seeing if it makes a difference...?
depending on the main dealers i would've thought they'd check UJs and such
if with the front prop off it still makes the noise i'd look at the front axle, if it stops i'd look at the tcase output and the prop UJs...
also does your jimny have auto locking hubs?