driveline and u-joint/rear yoke question

Thanks, ill get some pictures up possibly tomorrow night, i was supposed to meet up with a friend of mine to setup the gears and true trac, something came up. As far as the parts i have no clue on the driveline. I did notice it has a smaller diameter coming off of the rear yoke that goes into the larger diameter driveline, there is some type of rubber insulator inside where the two join. Any thoughts on this? The rear is an 8 3/4 out of a 69 imperial/new yorker (best guess) not 100% sure 489 case 11x2 1/2" brakes.