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.