Lost established Locke as the mystic quite early in the first season - the mysterious man endowed with a miracle cure with an intense, preternatural connection to island. This connection was severed late in the second season (with Locke's archetype replaced, to an extent, by Mr Eko) as his faith in the island was tested and found wanting. However this episode re-establishes the link.
John Locke, I think, has always longed to connect with something or someone beyond himself, something greater: whether this was a father, a spiritual quest, a miracle cure. I don't think we've seen how he hooked up with the criminal weed growers of this episode, but it's not surprising that they were steeped - at least superficially - in religious ceremony.
There was a major plot hole in the flashback: I didn't see why there was a need for the sting operation. Obviously the police knew about the drugs and the purpose of the tent must have been obvious, so why put a man undercover? The real, dramatic narrative, reason of course is to create someone that has a trust/betrayal relationship with Locke, but the "in game" necessity was threadbare.
Boone's appearance in Locke's psychedelic dream sequence was another meeting with someone from the past (a recurring theme in the show), though this time (as in Hurley's case) there's a semi-plausible explanation, ie drug-induced hallucination. A paranormal explanation for Desmond's precognition does however seem to be necessary - add another element to the Lost "stealth SF/F" pile of tricks and shticks.
I wonder if Hurley's line to Desmond about the failsafe key being "kind of convenient" was an in-joke? Because it certainly was a deux-ex-machina to pull last season.
Overall, "Further Instructions" lacked the punch of the better episodes; it was watchable, but a place-holder. So far this season, the show is one for three.