clunk clunk in transfer case

I was messing around a while ago out in the dirt and I noticed(it was hard not to)a clunk(metallic)coming from the bottom of my Jeep. It's got the TF999 and 231. I had a buddy drive it on a mild incline and stop so I could stick my ear up to it when he applied pressure on the gas. It clunked when it was under load right at the end of the t-case where the front driveline is connected. probably not the smartest thing I could have done to hear the noise but I'm alive and well. I changed the U-joints, front and rear, and the noise is still there. It has more often when I'm in reverse than in drive so that tells me it could be the sprocket at the front driveline which connects to the drive chain. Any ideas?

Tim