Chris. Look at the color diagrams in the FSM. See what circuit and pressure conditions are needed for reverse.
I recall going into reverse from dead cold in fast idle is very high pressure. Enough shock to break things. Cold weather tips were to put in N briefly (to get the fluid spread around) before reverse.
Point is it seems like something changed with the cooler change. Definately worth the effort to figure out the required circuits and what could go wrong. Then maybe you can come up with some simple tests or investigations.
OK I'm out ta here
PS There's a whole website with PS digitized
