My thighs tend to rub together as well. And when you run trails, they're not getting any smaller due to building more muscle :) For awhile I just used BodyGlide, but it wears off after many hours of running.
So I tried various compression/athletic "under" wear. The trick for me was getting them long enough that they don't ride up the thigh as your legs move.
Personally, I like Champion "Tech Performance Long Boxer Brief":
https://amzn.to/30e2vGl They're very inexpensive but do a great job, just as good as expensive brands, in my opinion.
On ultras I use BOTH BodyGlide AND those briefs and have zero chafing since. And I've run multiple 50's and a 100 with that combination.