Baaahhhh Trans!

[SourTooth307]

Hey everyone. Something seems to be up with my trans, maybe you can help me diagnose it....

Took a 300 mile round trip yesterday in my 84 K/10 and noticed what I would call a rare problem. Aside from the afore mentioned clunking into Drive or Reverse my trans is a perfect shifter. I happened to notice there is a minor slip going from 1st to 2nd ONLY when moving along at a crawling pace with very light throttle. I saw this happen when sitting in bumper to bumper traffic driving past Philadelphia. If you're taking off from a dead stop and have enough room to give it a normal amount of throttle, even in a 25mph zone there is no noticeable problem. I also noticed if you do this exact kind of driving while not in the Overdrive position (regular "D" on the shifter) the problem disappears completely. I had it do this once before while sitting in traffic near Queens, NY so I pulled over and checked the trans fluid which was a quart low. The problem seemed to disappear after topping off the fluid with a fresh bottle of Dex/Merc from Advance Auto and driving for a little while. The trans fluid level is currently fine, but the fluid itself looks a little on the dark side. My head is thinking it might be time for a trans service with filter change, but I'm unsure if this is the beginning of an internal failure. Anyone have any ideas what kind of trouble I'm looking at?