Got a Tranny Question for you guys

Ok so I'm a little stumped to this one. I recently aquired a 99 Durango 4x4 5.9. I know the trannys are notorious for being temperamental so here's what it's doing. In drive just accelerating normally it goes through the gears fine. If you get on it even a little bit, 2nd gear goes away. So I went through the gears manually and 2nd gear has NOTHING.????? I dropped the pan and fluid and filter was dirty but not burnt smell at all. Maybe a pressure governer? Overdrive button is not working either but that could be another seperate issue. But why would it shift through all the gears under normal driving, but free revs when you kick it down or nothing at all manually dropping it in second? I'm stumped and before I start throwing a bunch of money at this sucker I figured I'd ask some of you guys that probably have or had a Durango. Thanks guys
Rick