After checking out the woodchuck pictures, I gotta say it definitely looked more like the woodchuck than it did the squirrel. The tail was exactly the same. Based on the size and color variations described in the links you posted, I'm about 90% sure it's a woodchuck.
I have two hesitations about it not being a woodchuck, both of which could have explanations. It was late December and woodchucks hibernate (but
TX is warm and it's been a very warm winter so far). The second is that 2-4 kilos seems pretty big compared to the size of the one I saw (but it could have been an adolescent).
Maybe I'll head back out there this weekend on the off chance I can get a second glimpse at it; and bring the camera
.