I Hope I'm not braking any rules with this repeat—but I hate to see you replacing the sensor when you don't really know its bad. The symptom you gave: "When I unpluged it, the car leaned out... almost wanted to stall." Does NOT indicate a bad O2 sensor. Nor does the advice of a mechanic that may not be familiar with this old K-Jet.

It indicates (most likely) that the mixture is probably adjusted too lean (or too rich as Mac suggested).

You are getting lots of good advice on changing the sensor, but think about it — the car ran worse with the sensor unplugged. So it was doing something (beneficial) when connected, It may not be working like new, but it did nothing to make your car lean out or want to stall.
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Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and still here)
New 144 from '67 to '78
Used '62 122 from '63 to '67