Do i need surgery for rotator cuff tendinosis and shoulder instability? i can't reach behind my back at all and am limited in raising my arm

Diagnose first: You first need to diagnose the true problem or injury before any surgical or non-surgical treatment can be recommended.

See details: It depends what the examination and MRI of the shoulder show. A pure tendinitis often responds to NSAIDs, physical therapy and possibly cortisone injections.

This depends ..: You need to make sure you have a formal evaluation by a medical professional, ideally an orthopedic surgeon to make sure this is no concerning weakness, nerve compromise or suggestion of dislocation or bony compromise. If it really is just a case of rotator cuff tendinopthy and impingement then you may benefit from physical therapy and a possible steroid injection. Surgery is a last option!

GIRD: Sounds like glenohumeral internal rotational deficit. Usually resolved with therapy