Is there a breather on the T-Case, I am loosing fluid from the rear output shaft seal, but it will only loose fluid while I'm driving, and then it only looses about 1/3 - 1/2 quart then the fluid loss seems to stop.

Thought that if the T-Case was developing excess pressure this could explain this

 

No, there is no breather tube on your t/case. You may have a scored output shaft that the seal can not seal.

 

There is a product called White Shepard, others have used it and it stopped their leak.