John,
Here's how I understand it:
D52 is 5200 pound axle and D35 is 3500 pound axle. The D52 has larger brakes and larger spindle that requires larger bearings and probably other different parts.
Believe when an axle label says "D52 capacity: 3500 pounds" meaning the axle assembly came with a 5200 pound axle with 3500 pound springs, 4 leaf, the assembly is rated at 3500 pounds.
Our LE2 came with axle labels "D52 capacity: 5200 pounds" meaning the axle assembly came with a 5200 pound axle with 5200 pound, possibly higher rated capacity springs, 5 leaf, the assembly is rated at 5200 pounds.
Believe Mountainman198 posted about this in another thread, too.