Is this a 4X4 if so and the knocking is coming from the front you have a bad constant velocity joint in one of your half shafts. If this turns out to be the case do not change out the CVJ, instead change the whole half shaft the part at NAPA online is a few buck less the 70 dollars, The CVJ is about 50 bucks. with the half shaft you get two new CVJ's and new rubber boots.
My Carbon foot print is 57 tons per year, and that doesn't count my fart gas.
I spend my carbon offset money on my 1995 F150 Flareside with a carbon spewing 5.0L V8.