Surging problem

Could be caused by one of the following below

Oxygen sensor.
Catalytic converter.
Fuel injectors dirty/sticking.
Mass airflow sensor/Airflow meter.
Throttle position sensor.
Crankshaft position sensor
Knock sensor
Manifold absolute pressure sensor.
EGR Valve
Fuel pressure regulator leaking or defective fuel pump.
Fuel contamination.
Foul/defective spark plugs.
Open spark plug wires.
Ignition coil/Coil packs defective.
Incorrect ignition timing.
Cap and rotor.

Note:If it doesn't apply disregard it and keep testing.

Sounds like the lock-up torque converter is kicking out. The clue is engine speed will go up by 200 rpm when you are in third or fourth gear and above about 45 mph. Try holding the brake pedal up with your foot. If the problem stops, readjust the brake light switch.

If it still acts up, try driving at a steady highway speed and hold the brake pedal down just a little. If the engine speed stays higher, replace the brake light switch.

What happened to engine speed when you held the brake pedal down?

Tapping, (or holding) the brake pedal gives the signal for the computer to unlock the torque converter in preparation for coming to a stop. It would seem that undesired unlocking is what you're feeling. Now the question is why is it unlocking while you're driving. An intermittent brake light switch is one possible reason. A bad spot on the throttle position sensor is another but you replaced that already. There could be a wiring problem to the TPS too but that should set a fault code and turn on the Check Engine light.

If holding the brake pedal up doesn't help, the next thing would be to connect a scanner that can display live sensor data while you're driving. Many of them have a record function that lets you slowly review the data when the problem occurred. When the problem acts up, you press the "record" button. Because the data passes through the scanner's memory, the recording actually begins a few seconds before you pressed the button.

When you play the recording back you can look at what changed just before the torque converter was commanded to unlock. If the command to remain locked up never turned off, it is not a sensor or switch problem; it is a problem within the transmission itself. That would not be very common.

I would have it test driven by someone from a transmission specialty shop first to verify it's really the torque converter unlocking. If it is, you can be fairly certain they've seen it before or at least will know what to look for.

I'm not a Jeep guy so I can't speak to the cross member issue, but from my Chrysler experience I have no reason to doubt the diagnosis. I DO know these sensors cost more than I remember from the '90s so with the labor to remove a cross member it sounds like they are being fair.

All right. One in a row! Happy to hear you can buzz off, ... Ah, ... I mean SAIL off into the sunset.