i would say def swap the fluid out and u should be fine. it should just be some moisture in the fluid if u have done some water holes.
here is a fact: Fisher snow plows use two types of fluid, originally it was an ATF, which they still use. Then the company wanted to start making there own instead of sending out a plow with a valvoline or mobil atf bottle etc. so they had there own produced which in reality is the same as ATF WITHOUT the added red color to it. So i would have to say seeing snow plows all over the US from top to bottom, (way up in canada where it reaches extremely cold temps) and the plows... not having the ability to warm up like a vehicle, still run without issues. Your traditional ATF should be absolutley fine in your climate area.
that is why i am leaning towards it being something such as water/moisture in the fluid or even a mechanical issue. (maybe even moisture on the linkage causing it to give you an issue.