I was under my X attempting to install the gas skid and notice a small amount of either grease or gas splattered around the rear driveshaft U-Joint. It was on the corner of the gas tank and the E-brake line coming out of the drivers compartment.
Can you smell it? If it stinks to high heaven, then it's probably gear oil.
Clean the area thoroughly. No go out for a drive to get the fluids nice and warm. After the truck has set for a while (15 mins.), go back and see if you're leaking fluids. Perhaps now you can trace the source.
I didn't think anything on our trucks are greaseable, but it was a thought. Hmm, take a look at the TC in the SM and look to see what the seals look like. You said it is a slip yoke? Maybe the center of the yoke is hollow.