Hello, Which category do I need to book to rent a grand cherokee jeep? Is there a way to be sure to have a big cherokee jeep the day of my arrival (19 days rental from August 31st in seattle tacoma airport) And what category for a dodge challenger?

Nope cant promise a Cherokee. BUT if they dont have anything similar size and class they will upgrade you to a Chevy suburban for free.

There is no way to guarantee a certain type of vehicle...you shuttle over to the rental car facility and go to the category of vehicle you paid for (i.e. "midsize suv") and pick a car that is available, at that time, in that category. People are constantly picking up and dropping off, so the facility cleans out and washes the ones that get dropped off and brings them up to the rental facility. It's fcfs.