New chain and new cassette. I didn't think it was the back end because I could get it to crack at the bottom of the pedal stroke. It probably is the spider but needs more resistance in place. Stupid bikes.
Always found I could identify my stupid Cannondale BB creak by putting force on the cranks when they were horizontal and holding the bike (ie not sat on it). Might help to isolate the cause.
How old/worn is the chain? I recently replacing a chain which was creaking under load - I didn't realise how old/worn it was...