Valium and xanax are benzodiazepines and are not tested for on a DOT drug screen at all. In fact, it is illegal to test for these substances on a federal DOT test.
The DOT drug screen looks for marijuana, opiates, PCP, amphetamines and cocaine. There's no such thing as a drug it's illegal to test for according to the DOT; they could test you for steroids if they wanted to. The reason they only test fpr those five drugs is that they're the five drugs most likely to be found in a trucker's urine. Considering that most drivers who do drugs are doing so to stay awake, why would any trucker do downers?