so I got a question

how important is the rear O2 sensor after the cat on the
1990 toyota pickup 22re ?

 

I'm pretty sure that it does not contribute to fuel management.

It's purpose is to monitor the condition of the cat by watching the difference between before and after cat o2 readings.

Engine performance should not suffer if it is faulty, or non-existant, but you will get a check engine light.

 

i'm getting a smelly exhaust sometimes not all the time trying to figure out what the causes is

 

Not likely to be because of the rear o2 sensor.

 

What does it smell like, rotten eggs, burnt carbon, fuel, coolant?
Sometimes the smell can determine a starting point.

 

smells like burnt fuel not really rotten egg smell doesn't smell like raw fuel I have no exhaust leaks no smell in the engine compartment