Update:
Received replacement XX1 rear derailleur. Installed it this evening back on the FS IP-036, still takes me a while to get a R/D setup, seems to be working fine. Most importantly, no creaking. Now let's see how it does on the next ride.
IMPORTANT NOTE:
The SRAM "Powerlock" master link is a one time use item, it does not squeeze together and release like the old SRAM "PowerLink" did. I ended up needing to get out my chain tool and carefully break the chain, being careful not to push the pin out all the way.