That looks more like grease being slung on the muffler from the u-joint.
It's right in line with the joint and mine has the same mark and the t-case doesn't leak.
Right after I grease my joints the excess grease gets slung on the muffler and I'll bet every YJ that gets the U-Joints greased has the same mark.
Originally Posted by doubleroost
Does that symptom look like this. I have fluid being thrown onto my muffler.
I do have play where the slip yoke rides the tail shaft but I don't think replacing that seal will fix that... will it?