Need Help With 2000 Jimmy Front End Noise

Hi, new here but need some advice/help.

Girlfriend has a 2000 Jimmy XLT. Within a year, she's had the hubs and ball joints replaced.

Then, one day, by mistake, somehow the car got into 4 wheel drive, maybe stuck between drives, but not knowing, she took a long highway drive for several hours. She was hearing a loud meshing sound, but thought it was the trucks next to her and ignored it for a while. When she came to a stopped, the truck passed and she realized it was her truck making that noise.

She pulled over, noticed that the car was neither in 2wd or 4wd (neither button was really pushed in), so she pushed 4wd first, then 2wd, and started off again. The noise was there for a little while, then stopped. The rest of the trip was about 500 miles, and she heard no noise. Every now and then, she'll hear it, regardless of whether it's in 2 or 4 wd.

She now hears the noise every time she turns the wheel, regardless of wehther or not the car's at a standstill or moving, and she's afraid that the transfer case may be going.

Any ideas? Certainly any suggestions would be greatly appreciated! This is the only truck she's really trusted and felt safe in for years. Wants to keep it but worries that she's in for some very expensive repairs.

Personally my experience is in BMW E30s (I'm a moderator for a large BMW DYI site), and American cars are sort of foreign to me ;). Layman's terms right now to get me started would really help.

Thanks in advance.