Question: What is the sticky residue that is all over my pecans' leaves, also on the driveway beneath them?

Answer: That's honeydew that has been secreted by aphids in the pecans. Similar aphid infestations in crape myrtles and oaks, among others, will result in the same honeydew residue in late summer. Black sooty mold will start growing in the honeydew, further disfiguring the plants. Your main means of controlling the honeydew and resultant mold will be to control the aphids with a general-purpose insecticide that is labeled for use on pecans.