Entraped nerve?
 

Whenever I try and exercise my left shoulder (any head), what feels like the rotary cuff seems to be getting 'caught' on something, and producing extreme pain.

It's making it impossible to work my shoulders.

Anyone have any ideas on if this is an entraped nerve? Or is my rotary cuff simply kicking my ass?

Have you checked with a doctor about it?

Doctors won't look at it; they only refer me to sports clinics.

But my medical plan doesn't cover it, and to even have them look at it is bling bling.

So you can obviously see my hesitation

More likely rotator cuff tendonitis or bursitis. I doubt nerve entrapment or pressure as that usually causes numbness/tingling type of sensation. Of course, the test of choice here is an MRI

Tendonitis/bursitis requires a pretty strenuous physical therapy regimen to improve. Sometmes even cortisone injections are needed.

Try an exercise regimen first, under the direction of a trainer- should be lots of range of motion exercises. If that doesn't help, then you really need an MRI to rule out a rotator cuff tear