The pinion gear is mounted to the drive shaft in the same manner as is the rear brake drum onto the rear axle shaft. It is a tapered shaft and it can be on there very tight.
Got to the web site of the Santa Anita A's of Arcadia, California at www.santaanitaas.org On the home page put your cursor on "Technical Reference" My name will drop down below, click on it. it will pull up a series of technical articles. Scroll to Differential\pinion gear removal and click on it. The article will show you how to get the pinion gear off the drive shaft.