Odd shifting

I've got a '94 V6 with an auto tranny and the other day I was driving home and it just didn't seem to be shifting right. I was just following the flow of traffic at a consistent speed, the transmission wanted to down shift as if I hit the gas, but I didn't. It also hesitated when it went to shift a couple times. It would rev up to about 2500, from about 1500, and take a few seconds before it would shift.

Also, when I hit the highway, it would drop out of overdrive, similar to what it did in the city.

Any ideas as to why its doing it and how I can fix it.

I know its not fluid related, cuz the levels are fine. I do plan on changing the filter and replacing the fluid in the not to distant future.

I've also done some checking on other forums and it sounds like a speed sensor issue. My only question is where do I find it? Is it something easily accessable or is it something that requires some major work to get at?