What I've read about composting says that the juglone breaks down in about a year. Might take a little longer since compost provides a warmer, bacteria-rich environment to do the job. Or you could apply compost liberally to the area the trees occupied.

Depending on where you live in MN, I might know someone interested (maybe a couple of someones!). PM sent. If you get no takers for the walnut here, you might try the GW Woodworking forum. It would be a shame to let beautiful wood like that go to waste!

Here is a link that might be useful: Woodworking forum