After Very Hard Braking, Transmission/engine Temporarily Stuck In 2Nd

First, car is 2000 Corolla 4-spd auto (well maintained), 56k miles. Trans fluid is good (Royal purple). Car had leaking left half shaft axle...so I replaced both half shaft axles in front with new ones from advanced auto parts (forgot the brand)

Sometimes when I release the accelerator while moving at 50mph, I can feel the CV axles "disconnecting" from the transmission. I feel this on my other corolla too so I guess it's normal.

The problem:

Sometimes, when I brake very hard and come to a complete stop immediately, the transmission is temporarily stuck in 2nd gear for 2 seconds (the engine is around 1000rpm instead of 650rpm normal). Then the car lurches forward slightly (or sometimes there is a slight slight slight clunk). After 2 seconds, everything is fine and the car is idiling at 650rpm.

My guess is the very very hard braking locks the CV axles in place while the transmission still "spins" in second gear and the computer realizes it and downshifts to 1st

Earlier this morning, on a slight incline (almost flat), I braked normally and the trans was stuck in 2nd gear for 2 seconds. I let go off the brake and then it started rolling backwards....like the CV axles were disconnected from the trans and were rolling! I immediatelly, braked, and pressed the accelerator to go forward.

Any idea what could be the problem?

Did these symptoms start after replacing the axles? Could be that they axles are not fully seated into the transaxle or there could be an issue with the transaxle itself.

Could also be a totally separate issue - worn mounts can also cause those symptoms you are describing. A heavily worn mount can cause enough movement in the drivetrain that it could bind up like that on you.

Also possible that the car is stuck is some weird mode - resetting the ECM and having it relearn driving parameters can help. This especially transparent if there are multiple drivers of the car - the ECM will "learn" based on their driving habits - when you drive, feels like a totally different car / car acts weirdly.