We will want to leave a vehicle at Whitney Portal and get a ride with our backpacks to Cottonwood Lakes trail head. Is there any kind of van service we can hire?

You ought to read Kurt's reply to your previous post in this same forum.

For High Sierra Trail hikers leaving a car at Crescent Meadow, there is a way back by public transportation although it takes a full day and part of next morning. From Lone Pine, Eastern Sierra Transit on M-W-F leaves LP at 9:40 AM & arrives Mojave at 12:05pm. Kern Regional Transit Route 100 leaves from EST stop in Mojave at 1:10 pm Mon-Sun to Bakersfield and arrives at Grehound Station at 2:51pm. Greyhound has buses leaving 3:05, 3:55, & 5:10 pm arriving Visalia Transportation Center approx. 2 hours later. In the early AM the Sequoia Park seasonal shuttle leaves VTC once an hour in AM for $15. Google for current schedules.