I can't think of any advantage of doing Xrays first or at all.

If it's Lymphoma there could be other organs involved, and it should be able to pick up any tumors. Question: Are any of the dog's lymph nodes swollen? Other symptoms?

If it's kidney disease then again an ultrasound should give them an idea as to the shape and size of the kidneys as well if not better than an Xray.