Wheels off the back of the ground is way too high.
Sometimes a campsite can be high in the front. When hooking up, you can't get jack up high enough to engage the snap up brackets. In that case, simply don't hook the bars up. Pull the combo out on level ground, then hook them up.
To the OP, yes, it is normal to jack the back of the truck up, sometimes pretty high. Keep in mind, though, that you only need to go up high enough to engage the chain on the snap up bracket. If you can hang the chain, then you are high enough to use the leverage tool to finish the job.
Richard White
2011 F150 Ecoboost SCREW 145" 4x4
Firestone Ride-Rite Air Springs/Air Lift Wireless Controller
2006 Sportsmen by KZ 2604P (30')
Hensley Arrow