I'm pretty sure that 3/8" bolt is the shift rail detent spring and ball. If you remove it, if it is the detent retainer, a spring, and a ball should drop out of there. If you removed or loosened it previously, you should drain the fluid down far enough that it doesn't leak out with the bolt removed, use some brake cleaner on a q-tip or something to clean the thread, then bolt , and use a little RTV on the thread to seal that back up. Another thing, if it is the seal, take a close look at the output yoke surface where it meets the seal, most of the time, these have become worn and pitted by the seal. You could likely save yourself a headache when you replace the seal by getting a speedi sleeve (not cheap... about $40 for a stupid peice of tin, but they work well) from NAPA. I usually use a little RTV around the perimeter of this sleeve when I drive it on, helps ensure no leakage.