I assume it is a 1500 series ???

If so the tranny is a 700R4

Gonna need to get that box out and on the bench.
There are several things that can cause the issues you describe....

The 700R is not hard to go through.....

If you want to get an idea.....get the rig in the shop and carefully drop the pan......

Good bet it has a lot of broken metal pieces in the pan (Shavings)

Gonna need to come out and apart....

If the case is not damaged the box can be repaired....

Keep us posted as to what you decide to do and or find.....

I'm very curious as to whats broken.......
Reverse input clutch (Up front just behind the pump) .....Low reverse clutch (At the rear of the box)

Why is moves in "N" ?????? Parts ground up ??????