Strange that it's only one direction. Whatever happens, keep us updated, and post photos if possible. I need to do my seal eventually too. It looks like a pretty involved job.

 

Minor update that again points to the seal. Decided to try the simple method, spray some lube and see what happens.
Tried spray lithium in the area where the seal meets the pinion and squeak was gone for a few days. Haven't noted any leak from the diff, so likely the seal.

It's "used to" being rubbed in the forward direction, going backwards would make it grumpy.

 

It’s the “U” joint going bad