(Yes, I Googled this to no avail!)
I circled Highvale many times before finding this lunar rift, so I decided to post my finding. Feel free to post more!
Highvale lunar rift is in the southeast corner of the zone, behind Highvale village. The area appears as a circle on the map.
This entire scene is filled with enemies, so walking around is tedious at low level.
I did find a list of the rift destinations based on the moon phase here. http://sotawiki.net/sota/Lunar_Rift