How do I replace a heating element for this built-in oven? Can I just unscrew a couple of screws and pull it straight out; or, do I have to move the whole unit out in order to gain access at the rear side? 
Here are your parts
Replacement parts for General Electric JTP27WD1WW Electric range | AppliancePartsPros.com
Shut power off to the unit.
This is very important because on many units there is still 120 volts on one side of the element even with the element shut off!!!
Undo the screw holding the element to the back wall of the oven.
Gently pull the element into the oven.
Disconnect the wires from the element and replace it.
It is a good idea to clip something (clothespin etc.) onto the wires so that they cannot disappear into the back of the oven before you get them attached to the new element.