If you're running the module at 5v now, you're good for about 500ft (that's what I've gotten at 9600 a few times, a few errors, but it still worked). and if you read the datasheet, you'll see that you have to run your data input line to the TX module at the same voltage range as your power input.
I've ran that TX module at 2.5v before and gotten 100ft without troubles.
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