Backhoe. You didn't say if the wire is in conduit or not, but 2' down will be easiest with backhoe. Also, would you want to put other things in the trench such as water or conduit for low voltage? Yes to either question makes the 12" backhoe the only clean way to go.
I've put in a lot of AC, low voltage, and water around my place and always used my 12" bucket on the backhoe. All cable is in conduit. No worries about damaging the cable (rocks or groundhogs), less worries about lightning damage. Put a tape at around 1' deep. If you ever have to cross a cable, you can dig, find the tape, and then use a shovel - you'll run into conduit and not the wire itself. I tend to go down to 2.5' (a little overkill).
Pete