A pinion snubber is basicly a piece of sqaure tubing with a rubber "bumper" That contacts the rearend housing near where the pinion exits the front of the housing.
Mopar used to have adjustable one's and I believe they used to help prevent axle wrap.But now that I think about it they might have just been used to prevent the pinion from hitting the floor.When the axle wrapped up under hard launches.
Someone else will know more....
Also I believe you don't have to have one.Millions of RWD cars are on the road that don't have them.
A pinion snubber is a bumpstop for your rear end that prevents contact between your rear end housing and the body, and/or contact between your driveshaft and the body or any underbody components it may contact should your rear suspension become compressed further than intended. The snubber (bumpstop) is mounted to your body and will contact the pinion end of your rear end housing under compression. I do not run a snubber, but it is not a bad idea to have one.