AWD - how to check if it is working. Lexus RX 330.

ntwrkr

Had car in for service, up on a lift and I asked the mechanics to check out a sound coming from the rear end of the RX330. He said he would start the car, put it in gear and spin the tires to listen to the rear differential. To his surprise the front wheels were spinning at speed, but the rear driveshaft was only turning about 1 revolution per second and barely turning the rear wheels. Rather than have him run up a large labor charge while he got educated on how these AWD units work, I thought I would check some forums in search of enlightenment. We do think a wheel bearing is the likely cause of the noise, but the no power to the rear wheels when all 4 are off the ground is a question to be resolved first. Any ideas?