There is an emergency repair kit in the truck, which includes a repulsor jack. You spend a few minutes digging, and make an opening large enough to fit the jack in. You activate it, using the prybar to help guide the boulder as it rises, and you manage to get it a couple of inches off the ground. The jack shifts when the ground beneath it gives way, but you manage to quickly compensate and stabilize the boulder. Leaving the lifting mechanism to its own devices, you move to Screamer's shoulders. You're worried about not stabilizing his spinal column, but there's not much to do while he's under the boulder.
Fortunately, the ground beneath the bounder was relatively soft, and his body was driven into the ground, which helped absorb a lot of the force of the impact. You manage to haul him out. He looks to have internal injuries from the impact, but his lower body is not completely crushed. His legs, however, are in rough shape. They are bleeding badly once you remove him from the ground, in several places and he has complex compound fractures on both legs, and major internal bleeding.