Dear Doug,
Kidney and ureter stones can cause back pain that radiates all the way down to the scrotum. Dehydration is certainly a risk factor for kidney stones, however, the pain associated with stones is often more severe than the back pain that you describe. Another process that needs to be explored in any patient that complains of vague pain in the perineum(the area between the scrotum and the rectum) is prostatitis. The symptoms of bacterial and non-bacterial prostatitis are very similar. Most patients complain of painful urination, increased frequency of urination, ill-defined pelvic or perineal discomfort, and pain with or after ejaculation.
After reviewing your symptoms, I feel that you would benefit greatly from a thorough history and physical exam from a physician. A physical exam, including a rectal exam, would be useful to either localize your symptoms to the prostate or to possibly find other reasons for your symptoms such as kidney stones. Your physician would most likely perform a urinalysis to screen for bacterial infection. After taking a careful sexual history, your physician may also deem screening tests for sexually transmitted diseases as necessary. There is no substitute for a complete history and exam by a skilled physician. Don