Front Diff Noise

OK, I know this may get bounced to another forum because its about a Grand Cherokee.
BUT, my mom has a 99 Grand and its front differential is making a humming noise. And its above normal noise levels.
She never uses 4wd. Lives in the south (Virgina Beach). And only drives it about once a week. The stealership says it need a new front Diff for 1800.00 bucks.
Does this sound right. I wish the damn thing was here in NY with me.
Any Ideas what to look for as the culprit. Anyone else have front diff prob's with their Grands.
Don't beat me up for posting it here. Just looking for solid advice, as I have gotten much about my XJ in the past.
Thanks.
Oh yeah its for sale too. Seriously. She never drives it and takes her Camry everywhere anyway.

My friend had the same issue on his 98 GC. The diffs seem to be a common issue with them. He went to a local trans shop and spent somewhere around a 1,000 for it to be replaced. I believe they also flushed the TC and rear diff for the price as well

What drivetrain package. The frt. shafts on some can transmit a lot of noise into the cabin. Normally more of a problem on full time t-case though. Give me a little more info if yo can. I've worked on hundreds of these.