I levelled 52-55 in Kuftal Tunnel. Good exp there if you have a party member who can use Dispel.

Are the 51-53s in town or in the surrounding areas? I'm not sure what you could fight for good exp out there at level 51. Lesser Colibri were hitting me for pretty good damage at level 54, and I was under the impression they were the weakest enemies in the expansion area.