Solid axle means just that, the axle is one long and solid piece. Most vehicles have independent suspension, which means they have a-arms, cvd's, etc... A solid axle is like what is on the two trucks pictured above, an LB, Clodbuster, wheely king, basically any crawler. A solid axle is what the axle is on the back of a full scale truck, and independent axle is like what's in the front of a full scale front wheel drive car.