well, folks, I have now spent over 5 hours in the past two days talking to Motorola repair support and have not gotten an answer to this question. I believe that the only difference is in provisioning but I really want to make sure before I replace this screen. I am begging, if anyone knows this answer to let me know as soon as they can.
Hej there, I have the exact same problem - broken screen on a xt908 and theres no information out there on whether the xt910 screen unit works or not. Since I found an offer for some 55 € i just went ahead and ordered one. In about a week or ten days I will try to replace the screen and let you know if its compatible or not! Wish me luck
Alright, everyone - IT WORKS!
I mainly followed the guide from ifixit ( Motorola Droid RAZR Display Assembly Replacement - iFixit )
and now I got a new, unbroken screen So Xt910 and Xt912 parts are compatible with the Xt908 (as I expected). The phone rear case is a bit different so the replacement procedure is a bit different from the one in the guide, but it's all doable. Check the result yourself:
The only visible difference is the Motorola-Logo in the top, which was black with silver writing, not silver with black writing. That might be a color-specific difference, not a model-specific one (the new display + front frame was for a black model, i'm not sure but I think the old one was for a silver/gray one).
Best of luck to anyone out there who also wants to give it a try!