In order to get uptime from Pfsense via SNMP you have to enable all the snmp checks, including "host resources"

Then query oid 1.3.6.1.2.1.25.1.1.0 and you will get the uptime value in centisecond increments (10 milliseconds)

This also seems to work with engenius access points and WiFi distribution systems.

Your're welcome :)