Ever since I installed the stern power ring I've had a very loud whine come from the rear differential. The sound isn't what bothers me ,I'm just not sure if this is normal or signaling something wrong. Any ideas?

 

It is possible that you are hearing more noise than normal due to harder materials used on the mounts. I have replaced standard rubber bushing mounts with polyurethane types only to find that there was more noise afterwards.
Contact the manufacturer & see what they have to say. It is possible it is completely normal.