96 Ford Windstar

I bought my 96 Windstar in early 2004. About a month after I bought it I noticed there was a problem with the speedometer. It bounces around. The faster I go the more it bounces and it is off a little. Ie: when it's reading 34 I'm actually doing 30, when it reads 67 I'm actually doing 60. I had been told the speedsensor was causing the problem and that it is located on the transmission. I bought a new sensor but could not find it on the transmission. That was 10 months ago when I tried to replace it. Now a new problem has popped up with the transmission and I would like to know if the 2 problems are related. All of a sudden the tranny will not shift out of 2nd into 3rd and I cannot do more than about 20 miles per hour. If anyone has seen this sort of problem before or knows anything about it I would greatly appreciate your help. Thanks in advance.