Your year is close to the time they switched from cable driven speedo to the electrical version.
By the description, sounds like you have a cable driven one.
Easy to replace, just have to take the dash 1/2 apart to pull the instrument cluster (I think), then the rest is under the cab.
Good time to change out the radio if that was ever planned <img src="/forums/images/graemlins/lol.gif" alt="" />

Todd

1986 Pickup extracab longbed "Woody edition"
SAS - 33's, 5.29's, lockright rear.

02 IS300 SportCross - my "DD"
94 2wd v6 extracab
04 Outback Wagon- wife's/family car