Rear o2 will match the front allot of the times of the motor is extremely lean or extremely rich.. try cleaning the maf and check for vacuum leaks. To eliminate a bad brake booster unplug it's vac line and plug it. If the car runs good after then replace the booster. You can also use the fuel trims to do a cyl balance test. You can also inject propane into the intake hose by the air filter and see if the fuel trims come down, if they do then it's a fuel problem or possibly vacuum leak.. inject the propane and see what happens, check for vacuum leaks and bad brake booster and do the cyl balance test.. you should be able to figure out the problem by doing those things. If it's a series 1 or 2 3800 there is a real good chance the Mass air flow sensor is bad. They are known for bad maf! But only replace with a acdelco maf! To many after market maf been bad out of the box.
I know this is a old thread but I figured I would put some knowledge out there for anyone with those cars still
