double sheer means plate on both sides of the bolt, just like your lower mounts, you need to support the other side of the bolt, otherwise the bolt will just work back and forth until it brakes.
frame extentions:
if you run a gusset from the start of the stock frame, all the way to the end of your extensions. basically like what you have already done, just bigger, all the way to the end. that will help. maybe even run 2 of them close to the edges and cap it. think of it like a door wedge on top of the extention, big end at the truck, small end at the stinger.
and as far as everything being low, its called approach angle, the further away from the tire the "overhang" of the truck is , the more it will hang up on stuff. it will get to the rock before the tire has a chance to drive over it. and it being low makes that matter worse. the stock bumper may have been that low at stock height, but it was allot closer to the front tires.
think of it like this. if the stock bumper was 3 feet in front of the truck, what would happen if you tried to drive up a steep driveway? now pull that bumper back in, even tho its the same height off the ground, the fact that its closer to the tires means it will not hit the steep driveway first.
hope this helps
-Lance