I believe you 2000 Sabre has 4T65E transmission. Put on in my Pk.Ave easy and no harsh shifts no more. Had the upgraded DEXRON VI (6) with 7 quarts on standby. Put in 6 drove til hot then topped up until on full line. Also I use LubeGard red bottle to keep fluid cool.
Here is a link to shift kit I used
TransGo | 4T65E: SK 4T65E
Before I did the shift kit install, I used Trans Tune Can to clean up any varnish especially towards the valve body end.
Trans Tune | Sea Foam Sales Company
You drop tranny pan and drop down accumulator to install shift kit. Transgo has good instructions and my photos from this page will then make sense, how stuff laid out on table etc.
4T65-E Transmission Questions Help
Hope that helps.