I have had this truck (01 GMC 2500 HD 4x4) for about 3 months, and absolutely love it. I have noticed that it leaves a couple drops of oil on the driveway at night. I have crawled under the truck and it seems to be coming from the back of the transfer case. What might this be? I don't really have time to get it to a mechanic to have it checked.

Check the case bolts for tightness. Sometimes for no apparent reason a bolt will refuse to stay tight and present a slight leak. If you can not find a torque, snug tight will surffice.

Allison transmissions have had some problems with cracks in the tail shaft section of the transmission. I saw one in our dealership that was having the tail section replaced. It is my understanding that a bad u-joint can cause a vibration in the driveline resulting in the crack.