What is the major difference between a "semi float" and "full float" solid
rear axle?

A. Semi float axle designs include air chambers that can help the vehicle
(for example, the original VW Beetle) temporarily float in water. Full float
axles have blown aluminum housings and are packed with foam. They are used
on military/specialized vehicles that need to remain buoyant indefinitely.

B. Semi float rear axles are attached to the car body/frame at three or more
points such as leaf spring shackles, panhard rod, shock absorbers, etc. Full
float axles are connected to the vehicle body/frame with only two large coil
springs.

C. Semi float rear axles have axle shafts that both support the weight of
the vehicle and transmit torque from the differential to turn the wheels.
Older design rear wheel drive passenger cars and most light duty trucks have
solid, semi float axles. Full float axles have axle shafts that only
transmit torque from the differential to turn the wheels. The weight of the
vehicle is supported by the axle housing (the steel tube and other structure
surrounding the axle shafts). Large trucks that haul heavy loads typically
have full float, solid axles. Front wheel drive and modern independent rear
suspension axle shafts are similar to full float, solid axles in that they
transmit torque but do not bear vehicle weight.