I know they are 8 mm, but the person before me didn't . The ones at the axle are good not the tcase though. Do I have to grind them or is there a trick

 

I had that same problem ...If you have a welder, I would just weld a nut on top of the bolt and turn them out that way...after you weld the nut on cool it with water and it should come right out....now if you don't have a welder, just grind them off and remove them after you get the shaft out ...either way you will have to get new bolts but that's the easiest ways...hope this helps...

 

Try using locking vice grips to turn them out, just clamp down tight on them, then replace with new bolts.

 

So I used vice grips an wd40. Once I broke them i could use the 8mm. Turns out that was causing my horrible vibration. Does the front axle spin even in4wd? My hubs aren't locked

 

WD40 do yourself a favor and find some PB Blaster, Liquid Wrench, or best yet Kroil.

Absolutely that will cause horrible vibrations. The front axle on most years is always spinning as XJs do not have locking hubs, 4WD is controlled only by the transfer case. 84-90 XJs have a vacuum disconnect system in the front axle which may keep the front shaft from spinning, but the rest is always engaged.

 

Is the double cardan joint hard to rebuild? Do they sell an assembled one?

 

Do yourself a favor and replace the weak straps with a U bolt. If you can't get the 8mm bolts off or break them off like I did, you'll need to drill the broken bolt out carefully using a titanium metal drill bit.

http://www.naxja.org/forum/showthread.php?t=1071951