You don’t need to remove the castle nut unless you are changing the LBJ, axle, or something else other than the shock/spring assembly. In the front there’s the pinch bolt, and the bolt that goes through the foot of the lower shock mount (just above the axle), in the rear there is the foot bolt only. and of course the nuts at the top of the shock.
For seized nuts/bolts try a penetrant like PB Blaster or the like, soak it and hammer at it. If it’s completely seized then you might have to torch it. Make sure you're leveraging the suspension to take the tension off the foot bolts.
There’s a suspension guide in the DIY section, or you could have searched more here, YouTube (EricTheCarGuy), or Google.