Yep that's the one. Basically you need to pull the speedo sender out then turn the drive gear and see if the output turns. If the output turns when you turn the turn gear and you can make the speedo move by turning the speedo cable, then the problem is in the box. This is not what you want.
The speedo drive, drive gear is drvien by a gear mounted around the differential. To get the differential out the entire gearbox must be disassembled.
The Australian delivered MR2's were all NA so they all ran S series gearboxes that are a different family to the E153 in the turbo. The ST185 and AW11 Super Charged will be the same part as the SW20 turbo.