There is no easy way to see inside the transmission, it must come out to get inside.  That is alot of work, nothing real difficult just work.  Both tracks, track frames, sprockets and final drives have to be removed so you can take the two differential carrier bear quills off.  They extend into the side frames and have to be removed to slide the transmission housing out the back.  Once out, the shift assembley comes off the right side of the transmission expossing the gears.  If you want to get into that I have the OC 4 technical manual that gives the rebuild proceedures for the trans, diff and final drives.   Send me a MP with your e-mail and I can scan it and send it to you.