Here are your parts
Replacement parts for MAYTAG MZD2768GEB Ref - SXS/I&W | AppliancePartsPros.com
Here is a service manual
Try forcing a defrost cycle, see Page 2-15 in the manual.
With this unit it may be just as easy to check the defrost heater and defrost thermostat with a meter. Then if both are OK assume that the defrost board is the problem.