The leaves of 2 of my tomato plants are starting to curl. The plants were very healthy, and all of a sudden this week they look stunted with curled leaves. This is the 3rd year I have rotated my vegetables and I didn't have this problem last year, but had it several years ago. The other 2 plants look very healthy still. What should I do?

There are several causes for tomato leaves to curl, among them physiological issues, herbicide exposure, viruses or less commonly insect infestations. From the photos, it looks like the plant(s) have physiological leaf roll. This condition occurs when there are unusually hot and dry conditions much like we've had most of June. The symptoms are usually seen first on the lower, older leaves where they have an upward curling, followed by an inward rolling. It is not known to affect yield or fruit quality. Managing this is done by maintaining consistent moisture for the roots, using a mulch around the plants, and avoiding over-fertilizing. The fact that 2 plants show symptoms and 2 don't can be varietal differences or differences in plant response. Chemical injury is another possibility however the symptoms would be downward rolling and twisting of the leaves, stems that are thickened and twisted, as well as symptoms of yellowing and chlorosis. I don't see that on the images. You would best know if a chemical herbicide such as 2,4-D was used near/around the planting.