Bart,
If it's your heater core, you would know for sure...it usually smells strong of antifreeze in the cab.
That "whatever it is" looks like your front axle housing, but I don't remember what is above there that could be leaking, & it's too cold for me to generate much enthusiasm for crawling around under mine.![]()
Hope you find what's leaking...maybe carefully examine all hoses under the hood again.