This past Saturday I drained the transmission and torque converter, dropped the pan, and replaced the filter. The old fluid was somewhat dark (not bad, but darker than new) and there were some metal flakes present, but not terrible. Not enough to really concern me at the time, anyway. I did not notice any unusual odor.

I refilled the transmission with the engine at operating temperature, running in park, and filled it to the max line. Went for a test drive and everything felt fine. Shifts felt crisper than normal, and the car chirped the tires between 2nd and 3rd (never done that before!)

The car then sat for 48 hours. No leaks or anything else unusual developed.

Last night we got in to go to the grocery store, shifted into reverse, and nothing. Revved it up a little and I get a faint "clunk" from the rear end and the car moves forward slightly, then moves back again when I release the accelerator.

Shifting into forward gears produces the same result. Faint clunk from the rear, and the car won't go anywhere at all, regardless of RPM. But in forward gear, if I wind it up past about 2000 RPM, the speedometer starts to jump up to about 40 mph, then back to zero when I let off the accelerator.

I siphoned about half a quart of ATF out of the dipstick hole and poured a bottle of Trans-X high-mileage in, then ran the engine until it was at operating temperature again, and verified proper trans fluid level in park.

The car still won't go anywhere in any gear.

So ... it seems amazing to me, but did I break something simply by changing the fluid and filter?

I've got a "spare" transmission in a parts 300SD that I'm consdering dropping and putting in this car. It's got ~80k fewer miles on it and seems to work fine, but I haven't had a look at the pan.

Is there anything more I can do to troubleshoot before I just drop the transmission?

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Lots of diesel W126s in the past
Current: 1991 Toyota 4Runner with an OM603