TENS machines work on a different principle and I have not seen any evidence that they actually work in regards to killing parasites. If they were effective, it would be all over this forum and there would not be any need for zappers. TENS units are made to stimulate muscles and nerves which is one reason that they feel stronger.
As for the strength that you feel, one reason that TENS units feel stronger is that they operate at a lower frequency that is easier to be felt by nerves than zappers do.
There are also some zappers that can be felt quite strongly and do have a low frequency mode if that is what you are looking for.
If you do get a TENS unit and succeed, we would all like to hear about it.