One thing I've not seen in the entire thread is Coolant Temp Sensor. This and the O2 sensor(s), upstream, are the two main factors for attaining closed loop operation. As has been stated, the factory O2 sensors seem to make the vehicle happier.
I know it's a few bucks, but I have a Scan Gauge II in my '96. I have it set to read instant and average MPG, coolant temp., and voltage. There's also an option to make it monitor the open or closed loop operation, handy if that's what you're chasing.
I also have an UltraGauge, currently in our Outback. It displays eight different functions (as opposed to Scan Gauge's four), and it always shows loop operation. It's interesting to see how often the engine goes open or closed, depending on where the throttle is.
There is a way to test both the O2 and CTS, available somewhere here on
. I've used them both in diagnosing my '96, leading me to replace both my four-year-old O2 sensor (it had become lazy, and wasn't a Mopar issue), and the original CTS, as it didn't work at all, thereby prohibiting closed loop operation. Replaced them both, and mileage, within a couple of tanks, was up to over 20 MPG highway (original sticker estimate is 16/19), on stock springs and 235/70 tires. Although, not a match to my '92, which gets 23 highway all day long. Not much friction left in a 200k motor, I guess!