welcome .. I'm no expert at transmissions but these guys who wrote this article seem to be.. i gave it a quick look.. have a look at this page of common problems and see if it isnt torque converter shudder TCCoA Transmission Article - Page 4
2003 3.8L V6 4R70W
Windstar Upper Intake with cool-air intake, MAC LT headers, BBK O/R H-pipe, and flow 40s, SCT tuner, FRPP Alum. D/S, 3.73 gears, Auburn HP limited slip, and Team Z LCAs, Eibach sway bars.
Best 1/4 time: 15.036 @90.92mph, 2.148 60'