Just for the record, since there isn't much info on this anywhere..

I pulled the servo lock, big rubber bump stop, and GM3 (general) module off a junk car to try to retrofit soft close to my 4.6is. As it turns out, the wiring harness is not the same, and there are 2 wires for the soft close that don't exist on the pre-facelift models.

These wires are part of the car's main wiring harness and run all the way up to the general module behind the glove box. So unless you want to rip your whole car apart for the soft close feature, then it's not going to work.

Disappointing, but putting it out there for anyone else who is thinking of trying.