I finally bought a rototiller for my tractor; it is a CAT II 1GN-180H from Hillman Implements
With the rototiller behind the tractor, I quadrupled the size of the garden so that I no longer had to skimp on space and squeeze too many plants together. The tiller pulled up a lot of roots and quite a few rocks as well, but after 3 passes the sod had been mulched enough and the soil was ready for planting.
All the rain and snow this spring has delayed seeding by several weeks in our North West zone, but it did turn our slough into a nice lake! Last year the canoe was dragging over the weeds which were about 3 feet thick. This year we have at least a foot and a half more water and canoeing was great.