Hi all. Same issue here -- about a year ago, noticed limited range of motion. Saw a shoulder specialist, who (in my opinion) diagnosed "adhesive capsulitis" solely on the basis of the fact that I was diabetic. X-rays and MRI came up negative. 2nd shoulder specialist recommended manipulation.
And yet... my shoulder isn't frozen. It isn't locking up, and it isn't increasingly limited in range of motion. Plus which, I'm not the traditional patient. I am quite athletic, well within my target weight, and have a good A1C (somewhere around 6). I run about 20 miles a week and lift weights / do strength workouts.
What I've noticed is that WHEN I'M WARMED UP (for instance, after a 5K run or so) I have much greater range of motion. I can do MOST of the exercises that usually test for limited range of motion. And I've discovered that my range of motion is limited only in two directions. Trigger point guy suspects (I'm going to misquote him here, so sue me) upper deltoid and supraspinatus (spelling)?
Anyway, I've had a LOT of success with trigger point therapy and some pretty aggressive massage / stretch techniques.
I'd be VERY careful of anyone who prescribes treatment when they can't diagnose the problem better than "idiopathic adhesive capsulitis". In my opinion, that's just giving a fancy name to "I don't know."