&.5 and 8.8 shafts are the same--same steel, too. Same parts.
There are two things at work here. The rear axle has two jobs. It's a semi-float, meaning the weight of the truck is carried by ends of the shafts. The second part is that the thrust of starting and stopping the load is also carried, in torsion, by the shafts. The he load in the bed is the same no matter what engine you have. It's not a problem for any Ranger axle shaft.
Now you have to look upstream. What engine are you trying to throw at the load? The Ranger engines run from a 2.3L to a 4.0L. Te Ranger shafts are in no danger of being twisted up by these engines, and the loads are acceptable to the shafts. The bigger motors, Ford thought, need bigger ring and pinion gears to handle the engine torque and the deccel and accel surges. That's all. drive smoothly and a 5.0 with a 7.5 axle will be fine.