Figured it out. My corp laptop (the one I can't map from) was switched from Symantec AV to McAfee Host Intrusion Prevention last week (they do these kinds of things without telling users - grrrr). UDP is now blocked for all ports to and from untrusted hosts, which was not true when I had SAV running. Mapping attempt is blocked starting with the first UDP packet to port 137, so my laptop can't even see the DNS much less map it. I figured out I can turn the firewall off, map the drive, and still access it once I turn the firewall on again, so I at least have that to fall back on although its a pain.
They have also password-protected the configuration of HIP so I can't add my DNS to the trusted host list, so I think this workaround will have to do.