Reverse does not usually go like that;one day fine, next day gone. Reverse is achieved by simultaneously engaging , the L/R band(low/reverse), and the hi-drum. So with no reverse one of those is bad.
The L/R band is also used in manual low,but since the sprague is also working behind the scenes, automatically, there is no way to rule out the L/R band except by dropping the pan.
The hi drum is also used to make Drive/3rd gear, so if that works; by process of elimination, no reverse for you means the L/R band is mia. So it's time to drop the pan and check the band adjustment.
In all forward gears there is another clutch that engages; the Forward clutch . Chrysler calls theirs the rear clutch,cause it's the rearmost in their box.Yours seems to be working. But since your vehicle is creeping forward in Neutral, it is somehow still engaged, most likely,mechanically. There is no way for it to be so hydraulically, cause in Neutral the pressure to that circuit is dumped straight into the pan.
It may be that the L/R band is fine,but with the forward clutch engaged full time, with the selector lever in Reverse the tranny would be in locked mode; that is both clutches on and the L/R band as well. The result is no reverse motion.
That's my take on it........