The adventurers are moving through a relatively small rurual community, built up enough where land has passed down through generations and is scarce/valuable enough where it's become a standard to mark the boundaries of these lands with small standing stones.
Usually this is a pretty sleepy community aside from occasional bar fights, bandit raids and goblin child-snatchings; however, recently there's been a series of murders involving people traveling after dark in what is usually a pretty safe area. A fairly nototrious enfant terrible of one of the local noblemen has been jailed under suspicions of the murders.
Because of the various problems associated with frontier justice in rural communities dominated by a handful of rival families, the idea of having the visiting adventurers act as investigators/judges is attractive to all parties (except perhaps for the adventurers), especially if the group is associated with a regional power or has a cleric of a well-respected church as a member.
Shortly after the party agrees to investiage/adjudicate (if they choose to, of course), another person will be found murdered. This will cause a hubbub and possibly a full-on melee as rival clans clash over whether this exonerates the prisoner or whether this is just an attempt by his clan to clear his name. Things should escalate to out of control unless the party intervenes.
Careful investigation should reveal that the valley where all the victims have been found is split by the border between two of the largest and most ornery clans in the area and that there has been plenty of conflict over where exactly the border is.
Traveling the valley at night in a group will not reveal anything -- the ghost is ashamed of its crime and will avoid large groups, instead waiting to find groups of one or two, then accosting them to help move the land marker. All of the victims to date have refused as doing so as a great taboo (it can turn into a ghost, for one) and thus have been strangled. If the players manage to produce bait for the ghost, they can either attack when it reveals itself or help it move the stone (which should prove fairly difficult) -- when the stone has been moved, the ghost will move on from this mortal coil, although not immediately and it will try and take its secret to the next world by attacking those that helped it (it'll dissipate within a certain time period, is how I would do it).
About the Victims
If you want to use a traditional D&D ghost, the victims should probably be found horribly aged and ravaged (which may be attributed to the shock of being attacked rather than instantly interpreted by locals as ghost-sign). I personally prefer that the victims be found dead of strangulation, with heavy bruising on the neck (indicating somebody incredibly strong).