What years and models of cars/trucks/vans had the A518 tranny with the push button OD switch?

95 and earlier dodge pickups/ram vans. Not sure what the first year was but it was phased in during the late, late 80's I think.

OK thanks. Easiest way to ID it is the deeper pan right?

Easiest fool-proof way is to first open the drivers door and look at the date sticker on the door jam near the area beside the seat back. Then if it falls in the range crawl under and check for the 727/518 versus 904/500 transmission pan shape(if you're looking exclusively for a 518 and not a 500). If that checks out look back further and visually ID the large overdrive tail shaft. It's pretty obvious because it's so fat compared to a 3-sp.

Wouldn't there also be an OD switch on the dash?

Yes it would, however if you're digging through junkyard trucks with thrashed up interiors...

If the options sticker is intact on the underside of the hood, the installed factory trans will be listed there as well.

'90 was the first year for the 518.