2001 automatic trans. Is shifting timing different between having the shifter on 3 vs having it on D? Having it on 3 for me seems to keep RPM a little higher and more consistently at 2k - 2.5k rpm. I get a little better gas milage around these Tennesee hills too.

You mean aside from not shifting into 4th? I don't think it affects the shift points, aside from preventing the upshift into 4th.

It also locks the torque converter in 3rd, and it allows you to dip into the throttle a lot more before it unlocks compared to how the TC behaves in 4th.

The only difference I've found is that in 3 the torque converter will lock up in 3rd. In D it will only lock up in 4th. It's almost like it gives you an extra gear between 3-4!

Good point. I've also been told, but not confirmed, that the later years will lockup in 3rd with the shifter in (D) whereas earlier years don't.

My 95 will lock up in 3rd. Lower kpa in the map table will keep the timing high and prevents fuel enrichment. I've run additional timing on various years for a while now.