Play in u-joints or what?

Just bought this '99 4runner LTD last week so I don't know much about it. I decided to try the 4 wheel drive out while driving home tonight. I pushed the button on the left side of the 4X4 shifter. The indicator said it was in 4wd. While driving, if I let off the gas and then hit the gas again, it seemed like there was alot of play in the differential or something. I would hit the gas and then "bang" underneath. No other noises otherwise, just when I hit the gas from coasting. Is this typical? How do I fix this?