Yesterday on my way to work, my transmission decided to get stuck in first gear all of a sudden. I was driving for a few minutes completely normally, not pushing the car hard at all. As I came to a stop to turn onto a street, I put the shift lever into first and it went in a tiny bit rough, but nothing alarmingly unusual. As I turned on to the street, it wouldn't shift into second so I just rode out first gear until I could pull into a parking lot, where I later had it towed to a mechanic. Turns out the shift linkage and clutch assembly are fine, there's some sort of internal failure in the transmission that's caused first gear to be stuck. Any ideas of what it could be? Am I better off rebuilding it, or should I get a new transmission? And if I get a new one and all I'm able to find is the 6 speed without the LSD, how easy would it be to swap over my old LSD? Thank you if you're able to help.