That's weird. I use my Garmin ducer with the Ultrex and I do not get any interference at all. I used to get a ton with my Motorguide.
From what I understand, you really shouldn't be getting any with the Ultrex. There is a fuse inside it that I've heard of blowing, and that will cause you to get more interference. That's what was explained to me by someone that had it happen.