I suspect aphids but would need to see the underside of the leaves. If it is aphids, horticultural oil applied should take care of it, but you need to clean up all the leaf debris, double bag it and get rid of it. Another possibility is that the tree received some overspray from a herbicide. Santa Rosa plums are very susceptible to herbicides applied anywhere in the tree's "drip" line. Remember that herbicides applied on very warm days will vaporize and spread outside of the area they were sprayed. Attached is a good article on problems know to affect plum trees.

http://plantdiseasehandbook.tamu.edu/food-crops/fruit-crops/plum/

Thanks for the info! I will recheck the tree again, but I haven't seen any signs of aphid damage (I know what the buggers look like as I have plenty of black cherry aphids on my cherry trees!) No herbicides have been applied anywhere near the tree, which is at least 40 feet from the property lines (and I don't know of any herbicides applied by our neighbors.) I also forgot to note that this issue is affecting primarily the leaves lower on the branches; the leaves at the tips are mostly unaffected. i'll take some pictures of the entire tree and the leaf undersides when I get home. 

If the leaves also have brown or black spots, it could be brown rot which is controlled with Captan spray.