So I compiled em-7.3.8 on a FreeBSD-8.3 virtual machine. I am successfully able to load that driver during boot for the virtual machine and everything seems to work.
I moved the driver over to PfSense's /boot/modules, gave proper 555 permissions, and added 'if_em_load="YES"' to /boot/loader.conf.local.
My PfSense would hang when booting on "configuring WAN interface". ifconfig in single user mode showed the interfaces seemed to work with the new driver, maybe I need to reconfigure my interfaces for pfsense? I can try it, I'll do it tonight when I have more time.
What's weird is I'm pretty sure this NIC worked flawlessly with PfSense 2.0.1 when I used it over a year ago.