The front clutch or drum is also being used while reverse is applied, Which is where those snap rings could have come from. You could try adjusting the rear band, which the pans needs to be dropped to do. What you said about it jumping out of reverse sounds like the rear band is not holding to me. I dont think you can get to the parts you need to access by removing just the over drive unit on the rear of the trans. The way it was designed to come apart is by removing the front pump and pulling everything out through the front of the case. It's best to just repair it once and do it right the first time, instead of trying to cut corners.
2001 Jeep Grand Cherokee Laredo 6 cylinder Selectrac
$2400 installed new radiator 1 window regulator 2 oxygen sensors