Checking the tension of the transfer case chain
 

Can you check the tension of the transfer case chain while it's still in the case, through the drain plug or something?

When I push my X5 hard, towing or going into the mountains, I hear a high frequency chain vibration that seems to be coming from under the car and not the timing chain. It lasts the day of hard driving, but then goes away.

I have never felt anything like the chain skipping a tooth, but assume this is the first sign of chain stretch.

Has anyone tackled chain replacement?

'01 X5 4.4i
139,000 mi.