Hate to burst your MPG bubble, but I have a large hill that I go up every day at 40 MPH. If I use 4th gear my UltraGauge will show 14.5 MPG up the hill. In third it shows 12.5 or so. Lugging up the hill definitely shows more MPG. At a steady 30 MPH on level surface in third it will show low 20's and 4th will show low 30's. RPM's use more fuel plain and simple.
You are 2wd and the OP asking the question is 4x4 access cab. He explicitly asked "I am trying to get the best gas mileage without hurting the transmission." Everything with this motor has to do with load and a 4x4 system has much more loss then what you are driving. You will probably see a lot different results with 400+lbs less weight and a different drive-train. These engines/manuals with 4x4 perform much better at a higher rpm and provide better overall mileage then following the low rpm shift method of the v6. It is also bad on the bearing/rings to lug the engine and will probably be leaking oil past the seals down the road if done constantly.
The answer to his question is: just a low enough rpm to not lug the engine no matter what gear he uses. 1400 is low for any gear and lugging with a 4x4 Access cab 2.7L engine.