In theory, TENS sounds like a good idea.
However, the effects are mild (at best) and very short-lived. The high frequency mode, for example, is not very good at overriding the signals being sent to the brain and the effects stop as soon as the current is turned off.
The low frequency mode is slightly more effective, but:
It does not produce many endorphins.
The endorphins that are released, are broken down by the body relatively quickly (using an enzyme called endorphinase); their effects are therefore short-lived.
The body develops a self-limiting tolerance to these chemicals over time, just as it does for any other drug (naturally occurring or otherwise), an effect known as down regulation.