That's why I posted this question. After reading all the responses and seeing a suggestion a little earlier about a trencher, I started looking into them. My resistance to them earlier was that I had rented a large walk-behind trencher for a small job and found it almost uncontrollable.
Now, I'm leaning towards a self-propelled, ride-on, 4WD trencher. I've seen some for relatively low prices on the Equipment Trader, and I suspect they'll be easier/cheaper to rent if I go that way. I also found reference to some folks who even use them for planting! For the price of a backhoe, I can get a trencher, a post hole digger with a large auger for planting small trees and shrubs, and have pently of $$$ left over.
Thenks, everyone, for all the guidance.