Hi opz! welcome to the club, I'm a pretty new member too but besides the point. Depending on where you bought it from I'm assuming a dealer since you did a trade in. I would defiantly have it looked at (on the dealers check book if at all possible)
There are probably multiple things that are going wrong. One likely is worn gears. I saw this in some of my past cars. The car would pop itself out of gear because the teeth were worn to the point that it couldn't hold on. Being that the tranny looked like it was opened is indeed worrisome.
There could be some thing going on with the clutch but I cant answer that one for you. Hopefully some of the more knowledgeable members will answer that.
If you have a question about a domestic 4x4 I can help a lot . as for imports I'm pretty new to the scene but learning as fast as possible.