Front axle question

I have a 98 XJ 4.0L Automatic with the "part time" 4WD.
I believe its a dana 30 front and a 8.25 rear. But can't be 100% positive.
Anyway, while driving in 2wd I hear a faint whine from the front end. It sounds like the vehicle is in 4wd. But I know in 4wd you can hear the front axle with more pronounced sound. This is just enough sound in 2wd to be noticable. It sounds like the front axle gears.
Mind you, there is no clunking, no banging, nothing like that. Just this faint hum of those front end gears clasping together.
Does anyone else have this little sound on their XJ?
Just curious. And if so, is there a way to eliminate it?
So far steering and drivability is perfect. No problems at all. Just a faint whine.

Thanks folks.

In your year, there is no front axle disconnect and no locking hubs, so the gears in the front differential are always turning. That's what you are hearing, and it's normal. The louder hum or whine you hear in 4WD isn't the front axle, it's the chain in the transfer case.

Have you checked the level of the gear oil?

Have not checked the oil level. I will. Thats good preventitive maint. If its low, what weight and brand is recomended?
Thank you.

I believe the factory recommendation is 80W-90 GL-5 gear oil. I'm running Valvoline 85W-140 synthetic blend.