I leveled a PrLLLL to 60 and it was pretty easy until I got to Rend in UBRS. That said, I'm in the process of leveling a druid as well to replace a lock for some of the tougher encounters where a pure tank would make things easier(magic immune mobs, Bosses etc).

Personally I wouldn't go moonkin since the crit buff doesn't do much for locks aoe'ing. I think going feral is the way to go. I'm wondering if the priest could go shadow for shadoweaving and still heal well enough for the druid to tank?