By default Xfce provides screen-locking via Xscreensaver. Here is how you change it.

Xfce runs a script called xflock4 to lock the screen, to change the default behavior just foist another script on Xfce! The default path settings for searching for this executable shows, that /usr/local/bin has higher priority than /usr/bin (here is the original xflock4 located). The rest should be clear!

E.g. to use xtrlock instead of Xscreensaver you just have to link to the binary:

On a multiuser system you may allow each user to use it’s own locking-solution. So just write a script that checks if \$HOME/.screenlock is executable and runs it or falls back to a default screensaver:

Save it executable as /usr/local/bin/xflock4 - done…