The line for analog is not a hard I-91 thing. The Mt Tom Site 18 (which is west of I-91) is a very wide coverage area site.
Look at the display, if you see signal strength bars you should be able to hear the traffic. I prefer to custom program the systems and do not use the Uniden zip code monitoring at all. I would recommend creating a favorite with only the 853.9000 frequency, no talkgroups, no NAC, and see if you hear anything. You should easily be getting the West Granville site from where you are. Use caution about too much antenna. The more B South sites you can receive, the bigger the chance of simulcast interference.
chris