Usually when recieving free stuff its good etiqute to go get it. Maybe even bring a present......
Yes this would require getting off your a$$ but slightly less effort then jacking up the rear end of your XJ and seeing if the wheel bearings are shot. You could even go all the way and pull the tires and drums and see if the seals are leaking, pull the cover and visually inspect the gears and maybe even pull the 4 bolts holding the axle shafts in and look at the bearings.
But this would require effort, it would probably be easier to have someone else take it for a drive and then have someone else drop off free parts. All you need now is for someone to install it for you, for free....
SHEESH!