As a physical therapist I would estimate between 5-10% of  my frozen shoulder patients are diabetic. The rest are split between idiopathic (no known cause/corelation) and secondary (preceded by a traumatic injury). Interestingly, the ones who try manipulation end up no better and sometimes worse! Obviously I'm biased towards conservative therapy for this condition because it works more often than not, and, nearly all of the patients get at least a measurable return of function, diabetics included.

I also find compliant diabetics have the best outcomes with conservative therapy (may have to do with compliance with exercise as well, but that's another study).Here's my most important tip for anyone having this condition: Find a therapist who has experience treating this...not all PTs are created  equal - some have skills that can make a huge difference in ROM and reducing pain. It's worth the effort to research and ask questions. Frozen shoulder exercise programs found on the internet can be valuable if the source is reputable. I've spoken with one author of a program who has such experience and his course is making a difference, probably because he provides interactive follow-up. Whatever course of treatment is pursued, whether it's PT, manipulation, surgery, etc..., make sure you do your due dilligence before jumping in.