The letter designation indicates the number of plys or actually the equivalent strength based on early bias ply tires.

D = 8
E = 10

There are tires that (obviously given the example) can share the same Load Index but have different Load Range designation. The most important is the former.

Basically the main difference between those two tires is one's ability to hold 80psi.

While I'm not a tire expert, seems to me that the D has to be as strong or maybe stronger to hold the same weight using less pressure. However it's taller sidewall could be a hindrance in stable towing.