Dark rangers are once banshee, so we can assume that they have banshee powers :
-charm (DR) is an upgrade of possession (Banshee), control undead (DK) is a form of charm
-black arrow (DR) is a curse on an arrow, silence (DR) is a curse, and who has curse? That's right! banshee has curse! strangulate (DK) may not be a curse per se, but works much like a curse.
-life drain (DR) = death syphon (DK)
-anti magic shell (banshee) = AMS (DK)
Therefore, DK = undead warrior empowered by liches (frost), banshee+necromancer (unholy). Now that he has banshee powers, he only need to be able to curse and carry a bow.