Are lower back pain and testicular cancer related?

Usually not: Testcular cancer may spread to the lymph nodes of the back but this is seldom painful. It seldom metastasizes to the bone. Your question is important and should be moot -- if there's a testicular mass it needs to be investigated, if a person has been treated for testicular cancer, low back pain isn't the normal way a recurrence presents. Best wishes.

Rarely: if you have a history of testicular can't and are experiencing severe back pain it should be evaluated with an MRI. Especially if there signs of cord compression such as numbness or weakness, difficulty walking etc.