Crepe Myrtles near Walnut Trees
Crapemyrtles demand full sun to do well, so ideally situate them far from big trees so they get as much sun as you can give them. Also remember to plan for their eventual mature size so that you do not need to prune them.
As far as the juglone produced by the trees, we do not find them listed as tolerant on this Extension article from Penn State: http://extension.psu.edu/plants/gardening/fact-sheets/trees-shrubs/landscaping-and-gardening-around-...
They don't show up on this list of 'sensitive' either:
We would err on the side of safety, and for best growth and results try locating them in full sun as far away as you can get them. The above link suggests 50-60 feet from the trunks of large walnuts.