Many thanks for the good questions, Miles.
Crawlspace, built on slab on grade. No standing water, but high humidity (75%). Probably coming up through the slab, which doesn't have a capillary break.
Front yard is moist, but no standing or running water.
The slab was poured on top of a gravel pad. But per the soil map, the native soil is a shallow (16-31" to bedrock) silt loam derived from glacial till. This extends up the hillside, too, where there are stretches of exposed ledge. Supposed to drain well, but not in my back yard!
There's 100 feet of hill above the house, all recently logged woods.
There is already a ditch around the back of the yard, dug before and separately from the house. (There was a farm here in the 19th century.) Soil from the ditch was used to create a berm on the downhill side. But water appears to be passing through the berm and/or coming up out of the ground in the yard itself.
Clearly the ditch needs to be fixed: Clean it out, check grade, waterproof. (How does one waterproof a ditch??) But if I'm dealing with more than surface runoff here, fixing the ditch won't necessarily fix the problem.
Any further thoughts?