Where are the speed sensors located?

Where are the speed sensors located? My 95 GC goes into limp mode every now and then. I've been trying to diagnose the problem and most of the times it seems to happen when I do a quick stop (like when a green traffic light decides to go red at the last minute). I can feel the tranny as it goes into neutral then it goes back into gear. When I start driving again it starts rolling very heavy, like its on second or third gear, then a few seconds later it snaps, jerks a little and swithches back to first gear. But then it won't go past second gear untill I shut it off and on again (limp mode)
If I perform the same exact scenario but coasting the van to a slow stop, the tranny will do it's job and switch to 1st gear the way it's supposed to be, no snapping or jerking!
Also, when I drive with the O/D switch turned off everything works as normal and it wont give me any troubles when coming to a quick stop. So my guess here is that the tranny is getting stuck in O/D and it won't release it at the specified speed that it should when slowing down to a stop.

bump... where have all the experts gone?

I will try to post images for you. Input First.
http://files.automotiveforums.com/gallery/watermark.php?file=503/356512input.jpg

Output

http://files.automotiveforums.com/gallery/watermark.php?file=503/356512output.jpg

Hope this helps.

Scott

Thanks a whole bunch...

Before you spend a day looking for the speed sensors like me, Verify that your van has the 4 speed overdrive transmission. A parts guy sold me new sensors and never asked if I had a 3 or 4 speed trans. Turns out this guy at advanced auto who was on the ball set me straight. there are no speed sensors in the 3 speed!