I was probably ripped off if book time is 3.2, but oh well. Sometimes you have to take one to the gut. So, the compressor DID die (I know the difference between the typical electrical whine of the compressor vs an imminent failure). It lasted about two months with the ear piercing sound, but cooling just fine. I really thought it was a bearing inside the compressor, but the tech said no - it was something electrical.
Found a compressor from car-part.com and it was $128 shipped. My Tahoe has over 287K miles on it. It doesn't deserve a factory new compressor. It'd only make the rest of the truck feel envious.
Total to have the mechanic install (he's a hybrid specialist, so he knows about the POE oil and electrical compressors - he does a lot of Prius'), evacuate and charge was $1230. Total out of pocket around $1350. Still a heck of a lot better than $4k for a new compressor and installation. And, considering no one else would install user-furnished used parts, I'll be happy.
Whine is gone, air is cool, and Florida is tolerable once again.