I sent a response below. I think your gut is right.

I developed a right frozen shoulder, as they call it, after I went off prednisone. It added to the stiffness as per the excellent overview of withdrawal by @tuckerp below. I read many posts about not tolerating pain and stiffness but I think you have to, as long as it is manageable and not at all debilitating like full on PMR, if you want to avoid permanent damage from the drug. Six months after I am still improving. My right shoulder has been getting more open and usable from Pilates and swimming. The stiffness is variable. Some days very good legs and other days stiff and vice versa for arms and shoulders. Never good together! But it is a slow improvement. The dreadful fatigue seems to have almost fully lifted so my body must be getting better. I have emerged with an intolerance of eggs though.