Transmission won't go in reverse.

More as to what @Lefty71 said;
If the manual valve is not on the detent, but is stuck or positioned between gears, it will act like a huge fluid dump back into the pan.
This can happen;
thru misadjustment of the shifter, OR,
thru an incorrectly sized external shift lever, OR
the internal shift lever having dropped down, out of engagement.

But If it turns out that those are all OK;
Reverse works by applying fluid pressure to the L/R band, AND simultaneously, to the High-Drum, which Chrysler calls the Front clutch.
So, After the basics are done, regarding the mechanical linkages, and it still does not work, the best thing to do is run a pressure test, to see if the hydraulics are working.