Here is what I have, and I am pretty sure (just not 100%) that these rails came out of a '80s Mirada (because that's what the buckets came out of).
Not the best photo, as I didn't want to bother with macro shots on the camera and there just wasn't much room for the built-in flash to light things up nicely:
But as you can see...there is no linkage of any sorts between the inner & outer rails. I have to say though, now that I've tightened down the seat rail nuts there is almost no play...in fact, enough so that I do not think I'm going to worry about this much more. Ideally, I wish Mopar did put a linkage (rod or steel line) between the 2 rails...it would absolutely prevent any type of movement.
I had completely stripped the old rails that came out of a Mirada...they were cleaned up, re-painted, re-assembled. I know for sure nothing is binding...my only worry was that I left something out, either inadvertenly (memory's not what it used to be...LOL) or because the donor car was actually missing the piece.
What I am curious about is the actual M-body bucket rail trim. If you had manual rails I'd absolutely pester you with a polite request to take as many photos as possible...but as Mike pointed out, the whole setup is different on the power seat.
If anyone does have a photo of a manual bucket seat rail arrangement in M-body, drop me a line. I can always re-work the stuff over the winter...