Theres a 91 turbo leggy for very cheap near me, was wondering how hard it would be to replace the 4wd sensor on the back of the a/t sensor. I would love to resto mod this car into a subaru rally show car, maybe "sponsored" by 555 cigarettes 

Like 1 bolt and its pops out of the housing... as long as the bolt doesn't break.

I'd only say it's a bad sensor if the circuit tests bad or the TCU is throwing a VSS 1 code (Code 32, IIRC). If it's code 24, its a Duty C which requires pulling of the tail housing (doable in-car). If the mileage is up there, I wouldn't be too surprised it's cheap due to a bad/lazy/dying 4EAT.

The guy said the sensor got stolen(not sure if actually or for another car), so I guess putting one on wouldn't be too hard.