I don't know much about how to troubleshoot this sort of thing. At this point, I'm guessing it might be the antenna. There are a couple of places where the coax is cracked -- I wonder if the line is sound. Perhaps after the holidays I'll have some time to find a local ham who has some time to help me troubleshoot it -- swapping out my antenna for a known good one, trying my antenna on a known good transceiver, etc.
In the mean time, I can listen to my hearts content on 2m and 70cm. If I'm feeling adventurous, I can try to get someone to talk with me on 70cm. I managed to do that yesterday afternoon as we drove into town and I identified myself like this: "KJ4AED, mobile. Driving through Burlington without 2m capabilities." Someone answered immediately saying that with an identification like that, he couldn't resist a short chat. It was pretty funny.
I found the manuals for both of the loaner radios I have now. I've read through the one for the mobile radio, but had already figured out most of how it operates. I've briefly scanned through the one for the HF radio. Getting that radio working is going to be another candidate for finding a local ham to help -- before I start trying to get an HF antenna around here, I'd like to take this radio to a known good antenna and see how it operates.
Anyway, there are definitely fun times ahead as I continue to get more involved in ham.