I have a 93 ford 7.3 idi turbo diesel with the same problem and have the engine tore down to the crank. The other option other than the head gaskets (which would cause coolant on the heads themselves) would be a leaking oil cooler. I'm going to tear into mine today and see if its the tube between the mounts that's sprung a leak. The only other thing I can think of it being is cativation in the cylinder walls (electrolysis?) Good luck.