This is a quote from your original post.

"He has diagnosed the problem as a defective gear shifter module. $882 to replace new and $640 to replace with a used part...."

Correct me if I'm wrong, but didn't we find that the shifter in your ML is the early version, which uses the gear recognition switch, and not the later ESM type?

Also, I believe your problem began after the conductor plate was replaced, right? If so, the new conductor plate may be the problem.

Assuming that all the diagnosis/work performed up to this point has been done properly, the next thing to do is check the four signal wires from the gear selector switch to the transmission ECU. Depending on the position of the shifter, the voltage at each of these wires will be either >11 or <1. These inputs let the trans ECU know what gear is selected. If one of these values is implausible the transmission goes into "limp" mode.