I don't know why it slipped off the axel pad it just did. I'm not sure how long it's been like that.I don't have spring retainer clips. Jeep stopped including them pretty early on. Under what circumstance did the spring slide off the axle pad? That might be a whole other topic.
Why do you want to replace the springs? Mixing springs is generally fine. Maybe don't do it if the difference in height is several inches.