GM 14-bolt FF Rear End with 149k miles

Wikipedia, Locking differential
Previously known as the 'Gov-Lock'...unofficially

Eaton.com family of differentials and GM's G80 is the MLocker at Eaton
Eaton Differentials and Traction Control


MLocker page...am wrong about the left/right rpm differential...it is 100 RPM
MLocker

Eaton wrote:

MLocker

Eaton's MLocker® mechanical locking differential provides drivers best-in-class traction without the need for push-buttons, shift knobs or other intervention method. Utilizing a flyweight mechanism and self-energizing clutch system, the Eaton MLocker engages in low traction situations when a wheel speed difference of 100 RPM or greater (left-to-right) is detected. Automatic locking takes place within a fraction of a second, providing the driver with increased safety and confidence when traveling on wet or icy roads, gravel, mud and dirt. The mechanical locker is also compatible with existing anti-lock brake and vehicle stability systems, which simplifies platform integration for global OEMs.

MLocker Operation
During normal driving conditions, the MLocker functions as a light-bias limited slip differential. When a low-traction situation occurs that causes a wheel speed difference greater than 100 RPM, a flyweight mechanism opens to engage a latching bracket. The stopped flyweight triggers a self-energizing clutch
system, forcing a cam plate to ramp against a side gear. Cam plate ramping will continue to increase until both axles turn at the same speed (full lock), which prevents further wheel slip. When the need for improved traction is gone - unlocking occurs automatically and the differential resumes normal operation.

As for your Astro...don't know...your mechanic should know what a G80 is
and his shop manual should say it is a locker and no friction modifier