Rear axle, in a '55 T-bird

[dmsfrr]

I will humbly plead ignorance on this question...
With the rear wheels blocked on the ground and the FoM trans in neutral, how much should the pinion shaft / drive shaft turn? How much overall backlash is too much?

The rear axle assembly may not be original to the car. If it's had previous work done I don't know.
There are no howling, clunking, clicking or grinding noises. Pinion shaft seal & cover gasket both drip.

A good front-end & suspension shop worked on the car a couple months ago and suggested rebuilding the differential, for a notable price.
There may(?) be excess wear in the ring/pinion & spider gears but I haven't taken the rear cover off because I wouldn't have a clue what to look for. (besides chipped teeth & filings in the gear lube)