Drove car for on 1st gear at 75mph

You drove it at 75 mph? Getting stuck in second gear is "limp mode" that allows you to drive it slowly to a repair shop without needing a tow truck. Forcing it to go that fast is going to destroy the engine, not the transmission. That would not be covered under any kind of warranty because they will call that abuse. We all know Chrysler has built some really tough engines but they aren't meant to go 7,000 rpm. NASCAR race engines go that fast but they are intended to last only 500 miles.

To get the system out of limp mode you simply turn off the ignition switch and restart the engine. It has nothing to do with disconnecting the battery. In fact, once it defaulted to limp mode there will be a diagnostic fault code in the Transmission Computer to tell you why it did that. Disconnecting the battery will erase that code and you'll lose that valuable information. Disconnecting the battery to replace the throttle body is not necessary. Your mechanic understands that doing so will wipe out the stored fuel trim data in the Engine Computer and the modified shift schedules in the Transmission Computer. Both of those will be rebuilt later as you drive but in the case of the transmission that can take a few miles or a few dozen shift cycles. Some people whine and snivel that the car doesn't feel right, but most people will believe the mechanic when he says "just drive it". To avoid those temporary changes in the feel of the car most mechanics don't disconnect the battery unless it's really necessary for safety, or they use a memory saver device to keep the electrical system powered during service.

The transmission problem really shouldn't have anything to do with the throttle body. Regardless, there's two things to look at but you'll need to enlist the help of a mechanic with a scanner that will access the Transmission Computer. Most of them will. First you need to read the diagnostic fault codes. Once it went to limp mode there will be a fault code to indicate the circuit or system that had the problem that was detected by the computer. It's important that you don't disconnect the battery or let it run dead before the codes are read because doing that will erase them, then that valuable information will be lost. Most codes also self-erase after a certain number of engine starts if the problem doesn't occur again so don't wait too long to have them read. For engine codes that used to be 50 engine starts.

The second thing is to read the "clutch volume index", (CVI). This may have changed by 2009 but on the older cars that was a set of four numbers indicating the number of ccs of fluid it took to apply each clutch pack. An experienced transmission mechanic can tell by those numbers how much wear has taken place and how much life is left in those clutch plates. That should not be a concern yet at the mileage you listed. My first suspicion is you're going to come up with a code related to a sensor.

There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.