If I play Shadowcloaked Lurker with Last Rites, is it reduced by 2 energy?

Yes — but only for the copies that stay in the trash. The Lurker you're playing has already left the trash before its cost is determined, so it can't count itself (FAQ #8430). Each remaining Shadowblade Lurker in your trash still gives [2] off, so with one other copy in trash the cost drops from 5 to 3; with two others, to 1. This works the same whether you play it from hand or from trash via Last Rites (Rule 366.2).