So is the diff cup still plastic? with a steel nib around the outdrive? At a rough guess - your diffs are getting hot enough to affect the plastic integrity. Now you have a temp gun, run your muggy and when you measure the electronic's temps also measure the centre diff temperature. The centre will be working the hardest but the front/rear are inside casings, meaning they cannot dissapate heat as well.

I don't remember if these HD diffs were available when I bought diff upgrades. I ended up with nova rc diff cups front and rear with a slipperential centre. There still rockin' (mind you I don't run it much nowadays)