Diff "grease" of lube is a very high viscosity silicone oil. Its a really heavy version of the shock oils. I haven't seen anything like it at the hardware store. I have heard of people using high temp grease and packing there diffs. They claim to vary the hardness by how much grease they put into the diff. This may work well, but the problem is reproducing the effect when you need to rebuild, replace or inspect the diff. You are probably better off getting the diff oils and using them.