help w/ Yukon axles & thrust spacers

You shouldn't need to remove the spacers.

Those nuts can be a pain. A good impact wrench should work. Make sure you get the new nut torqued correctly. Are you switching because you're switching to a new driveshaft, or for a stronger u-joint? I would use a conversion joint unless I HAD to switch to the larger yoke for some reason.

As a not of caution, be VERY sure that the spacers are in place, or removed. Don't ever chance having one fall in the housing. If one is sitting down in the fluid, there's a good chance that it will get sucked up into the gears while on the interstate when you're driving 85. It makes a really neat *BANG* noise, and jerks hard, and puts a cool hole in the back of the housing. That was the only time I had ever paid someone to do that swap for me, and look where it got me.