Transfer case noise

[rellim78]

Just yesterday, I went and bought some U-joints to repair my driveshaft because I thought that those were the problem that I was having hearing a 'clunking' sound when I go from park to reverse, and reverse to drive. I went under my truck to see if I could find which joint was making the noise, but saw that when I spun the driveshaft by hand, the whole driveshaft spun about 15 degrees in either direction, with some rattling noise coming from the transfer case. I checked the fluid in it and it's full. Is there some other problem that I might be missing here? The truck is a 1987 GMC S-15 2.8L V6 4WD with 78k miles.