You will run faster times if you keep the engine in it's power band which means keeping the engine between 2800-3300...3500 tops. Have you ever dynoed your truck? If so, look at the curve. I highly doubt you are making more power at 3700-4000 than you are at 2800-3200.
I will give you a real world example...
A young friend of mine was doing the same thing as you, shifting at 3700-4000. Actually wasn't even going into overdrive. He was running 12 flats at 113. I told him he needed to shift sooner and as fast as he could. Next pass out doing what I told him to do, he ran 11.5 at 118.
I am in overdrive before the 1/8 mi