My A-833 Is stuck in Overdrive. Need advice/help ASAP, please!

So, this is for the tranny in my '78 Dodge Street van. I came off the freeway this morning and went to downshift from 4th gear/overdrive into 2nd. It would not come out of 4th so i got it to the side of the road and after no luck getting it freed up I called a tow truck and had it brought to my friend's place just down the street from me. My driveway was full at the time.

I have done a little searching around it sounds like it might be something I can access & fix by just removing the side cover but I have no experience with the A-833 at all, or the proper terms to use to help me search for the right answers. I really need to get it driveable again asap so any help would be greatly appreciated! The more details the better.
Thanks! IMG_20170424_081835629_HDR.jpg