I am having "lots of fun" with my Venture again. Looks like transmission vent, but I am not 100% sure is it that or something electrical.

First of all - yes oil is brown, which is not a good sign. Interesting part is that before yesterday I had no problems with transmission - I couldn't feel that it's about to go.

Then yesterday I stopped and stop sign, started driving and after about 20 meters it stumbled and lost power from transmission. Basically engine runs fine, you can shift in to any gear, but nothing happens. Park still works and doesn't allow car to move, Neutral allow for car to move, but you can even feel when shifting in to the gear that engine and transmission doesn't connect. There seems to be enough oil in transmission.

Do you think it's transmission? Perhaps some sensor? Anything else? Should I try changing oil?

I am no expert in these things... Any advice will be much appreciated.