I finally figured it out through some old fashioned trial and error. I set the output to "in 2" and that fixed it.
And yes, what I thought was the stereo signal on Reaper was, in fact a doubled mono track.
But now I'm kind of curious about something. I'm using LeCab, so how is Reaper going to be handling that? I'd set LeCab to both "stereo" and "individual", but, since I'm olny sending a mono signal, wouldn't both "cabs" end up being blended in each channel? And if so, should I then set LeCab to mono instead?
I feel a tech related n00b headache coming on....