Honestly replacing a T-case chain depends upon the damage done in the transfer case if it is actually the chain jumping teeth. I am not saying for certain that the problem is your t-case chain, just saying from experience when the problem can't be found at the front axle then it is probably the t-case chain jumping teeth. As far as replacing it: I have never bought a new chain, I just swapped t-cases that were good, but the chain runs about 50-100 bucks depending upon what t-case you have. If you have looked extensively for the problem elsewhere I would pull rear driveshaft, disconnect the speedo on tail shaft, unbolt t-case cover half, pull cover off and have a look at the inside of t-case. You will be able to tell if the chain is jumping by looking at the inside of the cover, you should be able to see where the chain has hit the cover when it jumps teeth. You may be able to see where the teeth are worn. It is hard to tell whether the chain is stretched by looking at it.
The other method I have of deciphering whether or not it is the transfer case chain jumping teeth is much simpler. You said it only made clunks or noise when you accelerated hard so take the rear driveshaft out, either drain the t-case or just let whatever fluid comes out run out and then drive the jeep with only front driveshaft. My xj would only go if I was very light on the gas when I did this because chain would jump so bad. Problem is when rear driveshaft is in the rear wheels are spinning too which takes the torque off the chain and front axle.
Hope this helps. Joe