Here's the situation. During most of Jan. and Feb. my balance rates were pretty high (about 7 to 8 on 2 injectors) with spikes to 13. After running several tanks of diesel additive, now my injectors are running less than +/- 2.00. Any explanation of what could be going on? Should I have a dealer do a diagnostic, or should I just be happy that things are going well?

Partially clogged....Good Idea to run the cleaner. If you are genuinely concerned about your injectors, go run the truck hard for about 5-10 miles.
Then go look at your balance rates. If the balance rates are high after that, then you have an issue (above +6 or below -6 in Drive is an issue). P.S. What diesel additive are you using???