Most likely you need a wdh. I'm guessing (not knowing specs for your tt) thAt your tongue weight is heavier than your hitch is rated for as load bearing vs weight distributing (specs should be printed on hitch) At 25 ft you can get away with using a sway bar or integrated sway control like Reese dual cam or equal-i-zer.
Sway is often times related to how your tt is loaded, you may need more weight on the tongue. If it was true sway (increasing oscillations from side to side). There are varying things it could be depending on what exactly it was doing but I would say, yes start with a wdh.