I don't get why manufacturers suggest to tighten the diff all the way and then back it off X amount. This is a recipe for disaster IMO and certainly 'fully tightened' is open to interpretation. If you tighten the diff up as tight as your hands will allow then you're going to damage the plates and potentially other components too.
I hope I'm allowed to post this here - there are a number of video's detailing how to build and set the diff and slipper clutch. Don't agree with packing the grease in personally but in general it seems quite accurate.
Diff building and setting
BTW this is not to say you have overtightened your diff, but when you mentioned the 'backing it off' bit, I thought this might be of help.