The process in which a relatively unspecialized cell acquires specialized features of a posterior lateral line neuromast mantle cell. (N.B. This may be development of neuromast mantle cell type or a set of cells of neuromast mantle cell type. This will involve the change of a cell or set of cells from one cell identity to another). Mantle cells are non-sensory cells that surround the sensory strip, separating the neuromast from the epidermis. Mantle cells secrete the cupula in which the ciliary bundles of all of the hair cells are embedded.