Hypothesis 1:  Tibia fracture of right hind leg with possible femur fracture of left hind 
leg.

Tibial fracture is supported by the fact that there is palpable instability of the tibia and the 
tibia does not appear to be continuous in the midshaft area.  The joints in this limb appear 
to be normal.  Supporting the left femur fracture is the swollen limb and suspect bone 
discontinuity from physical exam.  There is also crepitation and abnormal movement of 
the limb in the femur area.  These could all be caused by the blunt force trauma of impact 
with a moving vehicle.