Hey Josh,
You are right, 8.5.1 Base is not available for download online. We have recently EOLed ni.com/src and have replaced it with ni.com/downloads (you will notice that ni.com/src redirects you to downloads). I think during the conversion to the new system 8.5.1 Base was missed. I am going to get that put back online, but in the meantime I will private message you a link to download 8.5.1 Base. I don't want to post it here because it is only a temporary link.
You will need to uninstall the Full version then install the Base version.
National Instruments
