RE: Loaded servo inhibiting speed controller?
Hard to imagine that some AA batteries could possibly support the needs of 2 DC motors and a 133oz/in. servo. It's not so much the unloaded voltage of the batteries, but their ability to supply current under load, and how much (the 'C' rating relates to this.) I agree that a Spektrum receiver can support several decent servos - that's what their meant to do - but this would usually be driven by a BEC that can supply 5-10A continuous at 5-6V - way too much for your flashlight batteries.
I think your power needs are causing a 'brown out', and the solution (or at least an easy test) would be to plug in a more serious battery. At 7.2V (2S lipo), your Fingertech device takes 2+A, you may need another 2+A for your servo(s), plus whatever your motors suck up - let's say a total of 8-10A at least for testing purposes - so a 2S 1000mah 10C lipo might just cover it. 2S 1000mah 20C would be better, or 1800mah 20C - depends how much weight you can carry.
Alternatively, have you tested the servo using a servo tester?
Hope this helps...Chris