As long as it doesn't leak or burn oil then sure it can definitly help.
The 203 is pretty bulletproof so it may not be necessary but won't hurt. If the motor doesn't burn oil or leak then it is a good candidate for synthetic. A full flush for the trans would be very $$$ but a pan drop with 5qt of synth & a new filter would be good also if the tranny is in good shape. The axles for an offroad truck are the place you want it most. They take the highest torque loads & beatings from the drivetrain. Amsoil's Severe Gear is a great product for diffs. It helps prevent the oil from pressing out from between the gears under heavy loads or shockloads. For an offroad truck Amsoil also recommends using there tractor fluid for the power steering.
The 2 parts I would be most interseted in running synth is the motor & the diffs. Then if you can afford/want do the other parts with synth also.
Harley