I realized the Serv-U was unable to listen on port 22 with along with SSH, so I changed the SFTP listener in Serv-U to port 2222. This seems to have worked well.
For those who've dealt with this, is this the correct method? Any reason to keep SFTP on 22 and move SSH to 2222 instead?
Are these Native Serv-U users? or are you using other type of authentication like LDAP or Database Users?