Transmission trouble

[79-240d]

I have a '79 240D, and I think I lost the transmission today.

I was accelerating from a stop light, and right where the transmission should have made the 2-3 shift, there was short groaning/clunking noise and the engine was spinning free and there was no power to the wheels. I tried shifting it manually and nothing would catch. Fortunately, I was able to coast to an open parking spot to wait for a tow truck.

I had it towed to my mechanic, so they can take a look, but wondering what yall's best guess is about what went wrong. It basically felt like the shifter was disconnected from anything. I have been noticing some transmission issues in the past -- the shifts are a bit rough and recently, it sometimes doesn't "catch" when I shift into drive and I have to try again.

If the transmission has failed, anyone know the ballpark price for a transmission rebuild?

Thanks in advance for any thoughts