How do you tell if this dampener is installed? Is this something that can be added if not already there? If so, any suggestions on where I get one?

if you have the dampeners installed, your rear output shaft will have a large thick metal ring attached to it (between the case and the flange). you can't miss it. if you have an aftermarket 1310 or 1350, you most likely installed a new yoke and will not have this there nor could you install one. now, still on the back side of the t-case, you should find a thick round rubber disk incased with metal and it should be bolted on with a bracket off to the the side of the output shaft. here's a shot of what they would look like....

yey for hubs!