I'll try to help you figure this out.
I'm looking at a 1993 schematic, but I suspect it will be the same.
Two differences between auto and manual jump out at me.
First is that R/W (red with white stripe) wire, that is marked as Auto Trans only.
It was providing a ground through the Park/Neutral Safety Switch in the Neutral position.
It could be as simple as grounding that wire. Try that first.
However,
The other side of that coin is the R/LB wire that is labelled Manual Trans Only. It takes power from the Ignition Switch START position and puts it into Pin 1 of the Shift Module. That is the only time that power gets applied, in the Start position, so it must trigger a logic gate in the shift module.
Lastly, the speed sensor that you mentioned appears to be connected to the LG and LB wires. Having it hanging is a good thing as it will not be able to deliver any speed info, meaning it should operate the same as if you were stopped. That does set you up for a BIG problem if you try to engage 4low when still moving, so don't do that.
So, I suggest you ground that R/W wire and try it.
If 94 does differ significantly from 93, hopefully one of the fart smuckers will see this thread and chime in. I don't have any 94 info to look at. I believe it does differ because the speed sensor (magnet) for 93 appears to be in the transfer case according to the schematic.