We're heading on the JMT NOBO starting 7/17. Looking for someone heading SOBO same day to swap cars in Lee Vining on 7/16. Other logistics options are expensive and inconvenient.

Welcome David S. You didn't say... where are you starting your northbound hike? Horseshoe meadows? Most southbounders will likely end their hike at Whitney Portal.

You might consider leaving your car in Mammoth -- the road just east of Mammoth Mtn Inn has had long term parking in the past. You can then take public transportation to/from Yosemite using YARTS, and the ESTA (https://www.estransit.com/) between Mammoth and Lone Pine. You can hitch hike pretty easily in Lone Pine to your trail head.

My plan would be to leave the other party's car at Whitney Portal, and then hitch down to Lone Pine and up to Horseshoe for our start. We can give each other spare keys just in case we don't cross paths on the trail.