The upper oven is no longer reading the correct temp. For example, if I set the oven to 350, it will almost immediately ding, indicating that it has preheated to 350. I can leave it on at that setting for an hour, but a cheapen oven thermometer set inside will only read 150 or so. Any ideas what this could be?
Here are your parts
Parts for Maytag JJW8627DDB29 Wall Oven - AppliancePartsPros.com
It could be the oven sensor.
Below is a link take a look at 7, 8, 20 and 21.
http://www.applianceaid.com/electric-range-repair-topics.php
Thank you, Denman. The sensor was my first thought, so I did go ahead and order one. Your link will help me with removal, testing, and reinstallation. Thank you!
I will let you know how it goes after the new part arrives 
I finally received my new sensors (74030P046-60) and they are not the correct sensors. Mine are straight, and these are L-shaped. I believe the correct part number should be 12001656, so I will reorder and give it another try.