I have a 99 Grand Caravan with approx 126k on it. Recently the transmission has started acting up between 40 - 55 mph. It doesn't seem to be shifting properly. The tach will shoot up to 3 and get stuck for a while and it won't change gears then it'll drop to 2 and it'll change fine. It seems to do this more when you are gradually building up speed.

 

Trans has two speed sensors. Input (Torque converter) and output (Driveshaft) speed.
My 96 grand did the same thing. I replaced the output and all was back to normal.

Do the "key dance" (on, off, on, off, on) and watch the check engine light for codes.
The Chilton / Haynes book has all the code explanations.
Or, most Autozones/ NAPA stores will scan your rig for free.

Good Luck,
EZ1

http://www.autozone.com/

And search "output speed sensor" $16.99

Remove left ft tire, splash guard, and the speed sensor should be right in front of you.
Unplug it, CLEAN THE AREA, unscrew the old, carfully screw in the new (it's plastic threads), plug it in, put the splash gaurd and wheel back on and your done.

 

Need to pull codes out fo the TCM, because the key dance will only give you stored PCM codes and one transmission stored code P0700. ck the fluid level as well.