Your mechanic is wrong. the pinion doesn't have to come out. Just unbolt the yoke and the retainer. Actually a pretty straight-forward job.
Here's a blowup of the differential


Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.