are you talking high range or low? running around fire roads in low or low low it whines a bit. on the highway, etc in high range, no.
I had a CJ-7 once with a dana 18 in it and as it turned out the output gear was a cheap charlie from China, and it whined the entire time I had it, and eventually ate the gears. when I pulled everything you could see the horrific wear pattern on that gear
have you called Marlin? could be new bearings just breaking in, tight tolerances.