4x4 question
 

Hey guys,
Recently bought my first Blazer. 2001 LS 4x4 with 4 button switch. Got what may be a stupid question. While putting on new rear brake pads having a rear wheel jacked up, I put it in gear turning the rotor checking for trueness, I noticed my front driveshaft is turning. Is this supossed to be? Forgive my ignorance, I'm trying to learn about it. Appreciate any help.