There's a spring inside the seal, that might have come out of it's groove. I'm pretty sure that happened to me, and the fluid slowly came out the rear seal and I didn't notice it until the TC started growling...
You may be able to put the spring back in place (maybe also smear some grease on it as well to hold it in) or replace the seal without needing to remove the TC from the truck.
2004 GMC Sierra 3500 C&C, added 4WD, 6.0L gas, Extended Cab, SLE, Transgo 4L80e-HD2 kit installed
Whee! Dually Club Member #1109