1991 Subaru Legacy speedo

Transmission problem
1991 Subaru Legacy 4 cyl Two Wheel Drive Automatic

have a 1991 Legacy LS Wagon. 4spd auto, 4wd. Supposedly 121,000 miles. The speedometer has stopped working one cold morning and has failed to start working again.

I checked the speedometer cable, it is in good shape, and well lubircated. When attached to a drill, the speedometer work. Upon further investigstion, the drive that comes from the front of the transmission, that feeds the cable, does not appear to turn at all, no matter the wheel speed. In light of this, I took it to a shop, they tried to sell me on an electrical speed sensor, that is located somewhere on the rear end of the transmission. Hoping for anything, I had them try it. They ended up ordering the sensor twice, neither one would fit or match up.

I called a dealership, some distance away. The dealers mechanic figured it was the driven gear. Gave some vague directions, and said that it wasn't that bad to replace. Doing some further investigation on my own, I can not figure out how to get the old one out to put the new one in. So far it appears to me the only option is to pull the transmission, and take most of it appart.

Is there some other way?