In a perfect world, Jodi would have used her cell phone close to the time she disposed of the evidence, and then we'd have a record from nearby cell tower pings of where she was in the desert.
I don't think JA would stop in an area where she could be spotted by potential witnesses. Maybe that's what took her so long to get to UT. I could also see her disposing of the evidence in a lake or pond of some kid. I feel like that might be easier than burying it in the open desert, maybe.
Another thought, perhaps she was so concerned about the rental car during the interrogation because she was afraid of trace evidence from the disposal site. I think even JA would know that if LE found the evidence, and linked it to trace on the rental car, she'd be done for. Toast.