I have an 89 LeSabre 4 door FWD with the 3.8L V6 engine and 125k miles on it. The transmission winds out too high before shifting gears, and rumbles somewhat in overdrive. I am researching my options with possibly needing to install a used/rebuilt transmission, but how do I find out which transmission specifically that I need? I see price quotes based on part numbers but How would I find out which p/n my tranny is?

there should be an ID plate on the trans near the shift linkage. On this plate there is a 4 digit code.
codes 9bhh, 9wrh, 0wmh & 0bhh are a 2.84:1 final drive ratio, AC Delco #24212203
codes 9bwh, 9wuh, 0bwh, & 0wph are a 2.97:1 final drive ratio AC Delco #24212204
Hope this info helps.