I would use any of the many load cells available and wire the bridges of these (the bridges are quite good at eliminating common mode noise on longer wires too) via a cable back to the location of the 8051. At the 8051 you will need to have a full differential type of instrumentation amplifier becasue the output of load cells can be as low as 5 mVolts full scale.

Your biggest challenge by far will be to modify the axle mountings on the vehicle in order to mount the load cells correctly. In addition to that you also need to consider the integrity of the load cell if the loads on it are partly off the normal compressive or extensive axes of the load cell.
Mike Karas