This little thing is mythical. I THINK, on a 72 A body and others, it's a little cone shaped thing with a threaded stud that bolts onto the FRAME, pointing down, and the idea is the schnozz of the differential hits it when it rotates up too much.

One symptom of THUD going from neutral to reverse is worn out snubber.

Is that what it is and where it goes? Can one jack up the car to full bound and replace it in such position?

 

Without a better description or a pic, it sounds like a "pinion snubber" Someone probably installed it for " extra load" capacity. How far from contact is it at normal ride height ?
Its really a 50 year old "trick" for drag racing. I used 1960 Olds upper a-frame bumpers and cut 1" off the end.

 

In the '72 CSM, it shows what I believe is that bumper (section 4-1 & 4-8), but no part # or other info is given. It looks like you could jack it up via the frame to get at it. It is secured with a nut on the top side. Access under there might be a problem.

 

In reverse the nose of the rear axle rotates down, away from the snubber.