?

As mentioned in the initial question, the truck's A/C system isn't cooling while the truck is idling. At highway speeds, everything appears to be fine. Problem started suddenly a week ago.

I took the truck in for a routine oil change and asked the guys to check on the A/C. I was thinking it might have been a low charge due to a leak. The mechanic said he thought it was probably the compressor ($450) but the shop manager (knows me and knows I'm close friends with the shop owner) wasn't 100% positive his mechanic's diagnosis and wanted me to take it to a specialist. In the meantime, they noticed a leaking water pump and replaced it while they had the truck.

All seemed fine until two days ago. The truck idled for a couple minutes while I was stuck in traffic. Right about the time I got moving again, I heard what sounded like a belt slipping badly underneath the hood. I immediately shut off the the AC and the sound stopped simulaneously. So now I suspect it is indeed the compressor. I've got the mechanical ability to swap the part myself but I'm looking for confirmation that my theory is viable.