I can't really tell what type of axle that is, but if you take it to NAPA or most any other good parts store, they can find you a correctly sized boot to replace it with. Probably a better one than came on it. As far as getting the cup out of the diff, your rears MAY be bolted together internally. you'll have to get a light pointed in there and see if theres a bolt head all the way down in there. If so, you will have to take the other side off and unbolt them. A lot of people had problems with the Gorillas (and other brands)popping out of the differential and started bolting the two together in the middle.
'06 660 Camo, 11:1 727 Stroker, HSR 45 Carb, Alba Exhaust, programmable CDI, 30" Silverbacks on 14" HD2's, Teryx Shocks, Gorilla 2" lift, 17" ground clearance, Snorkeled, J-Strong Roof with JVC stereo, and four Dually D2's, Ridgid 6"E series light bar, Warn 3000 lb winch, Electric fuel pump mod, Hotrod sheave .