Is it time to replace my compressor?

[twinquist]

I am trying to get the AC working on my 2006 Scion xB (150k miles)

Originally, I thought there was just a leak in the system. I wanted to take on the challenge, so I went ahead and replaced all the major AC components except for the compressor. I replaced the condenser, evaporator core (boy, that was a lot of fun!), and expansion valve. After putting a vacuum on the system for 45 minutes to remove all moisture, I recharged the system. I then added a "shot" of UV dye and ran the system for 30 minutes. Looking around with my black light, I see NO signs of leaks. However, the AC is still blowing as warm as the outside ambient temperature. The only major component left is the compressor. One thing I found when looking for leaks is that the compressor chatters badly. Almost sounds like a really large, metal, squeaky hamster wheel. During recharge, I took note of the high side and low side pressure. (For reference, it was about 85 degrees outside here in Florida). The low side pressure was close to 50 mmHg and the high side pressure was 51 mmHg. The reading of 51 mmHg on the high side seems really low to me but I can't seem to find any specs out there.

I'd hate to just "replace and hope" with the compressor since it's the most expensive component in the system and laying under a car for an hour doesn't sound like much fun. Does my high-side pressure reading and the fact that the compressor is "chattering" point to the compressor as the problem? I don't know what else it could be...

Any/all advice is highly appreciated!