Transmission on my '83 380 SEC suddenly stopped engaging.

Other than very minor slippage that I've come to expect on an older vehicle, there was no warning of this. I put it into drive, and engine just ran free. Same with other "gears" -- no resistance whatsoever.

Fluid level is still topped, with no apparent leakage, so I'm guessing this is just an internal seal?

What should I expect to pay for a rebuild? Would replacement be cheaper? (Adsit has a rebuilt one for $1400) Any insight about best/worst-case scenario here?