front pinion seal on 2dr tahoe

i'm a little baffled here my tahoe has developed a leak in the front diff. pinion seal. new seal is a simple fix for the leak, but i have noticed that there is splatter like the front drive shaft is turning at highway speeds. I may be crazy but i didn't think the front drive shaft was suppossed to turn when not engauged? i jacked the truck up and rotated both tire at the same time and the shaft didn't move, i also turned the drive shaft by hand and it turned freely without the front tires turning.i guess my biggest ? is that does the drive shaft turn at highway speeds without being engauged? just tring to make sure i don't have another problem i should be addressing at the same time. or could i just be crazy?:crazy: