1992 gmc vandura 3500 auto tranny speed sensor

I have a bluebird micro bus it is a 1992 GMC Vandura 3500. At times, the spedometor will stop working and the automatic transmission will not shift through the gears. I am told this may be a speed sensor problem. Can anyone tell me where this sensor is located? That way i can take it out, get a new one and put it back in. It would help if anyone has a picture. Thanks.

Dom