Re: Tranmission shifting erratically at mid range speeds
The PCM controls the shifts on your AOD-E trans, and can do strange things when the voltage is not correct. Judging from your car's age and low mileage, it might have been sitting for a while, so I would suspect corrosion and not internal wear to be the problem.
Start with the easiest and cheapest tests first. Check for battery cable and electrical connector corrosion. Correct if necessary. Additionally, I would change the fluid and filter if it hasn't been changed recently. While changing the fluid, and while the car is on JACK STANDS, check the output speed sensor (OSS) and it's electrical connections. The OSS is located on the transmission tailshaft about where the speedometer cable used to be on older cars. The OSS sends info to the PCM (as well as your speedometer) and erratic shifting will occur if the OSS is not sending correct info to the PCM.
While the transmission pan is off, to change the filter, you might want to remove the valve body and accumulators for further visual checks. This requires a little advanced mechanical knowledge, torque wrench, a very clean work space, and patience. If you decide to remove the valve body, I am familiar with this transmission and will assist all that I can. These suggestions are just a starting point without, getting too technically involved.