Its the dropping resistor on the driver side, next to the coil. If you unplug it, it will set the transmission line pressure to maximum, severely firming up the 1-2 shift and milding firming up the rest.
Instead of unplugging it, go to radio shack and get a couple of the larger, ceramic 50-ohm power resistors, they are larger than the tiny 1/4-watt ohm resistors most people are used to. Replace your dropping resistor with one, or two 50-ohm resistors connected end to end, to firm up your shifts. I have been using 100-ohms for several years now and I like it. The stock dropping resistor is about 15 ohms.
Driving around with it simply unplugged, with line pressure max'd out all the time, is not good for long term reliability.
Yes, change the fluid. Dont do a power flush, or use anything other than atf. Just change it (several times if necessary) using the drain plug, or drop the pan and clean it (and the magnet), change the filter, and replace whatever drained out. The thing about not changing old fluid is a myth. Use either a generic Dex III/Merc fluid, or Mercon V. The Mercon V has different friction characteristics that can give a very slightly firmer feeling shift. You may not notice any difference though.
Again, if it were me, I wouldnt just unplug the resistor....
