95 rough

Another issue I am having with my 95 Supercharged Riv involves a rough running engine. I have read other people's problems posted here and the answers they have received and all seem helpful. The problem I have with all this is that I have owned other 3800's of this vintage in different cars and none of them have had any of these problems. Those cars and engines have all run exceptionally well and I have always told anyone who asked that the 3800 is bullitproof. But now I have one that is not. I had Bonnevilles, Oldsmobiles, and Buicks that all ran perfect. The biggest recurring problem I had with those cars was with fuel pumps. Spark plugs and wires were never a problem with those motors.
Having said that, This 95 Riv with 60K does not idle smoothly. The further from idle it gets, the smoother it runs. Just recently I started to replace the spark plugs and stopped after replacing the front. How difficult is it to get at the rear plugs? Does one work from under the car or what? I have a broken leg at the moment so my mobility is a bit hampered.
Any other suggestions besides reading the codes? And what kind of gas mileage should I be getting with everyday driving? No hot rodding.

We should know if your 3800 is supercharged or not.

The rear plugs are hard, but not that hard to reach. I have long arms, I usually reach around by the power steering pump and then have pretty good access to all 3. The #6 is the hardest because it's furthest away, but you can probably get there from under the car if you can't do it from the top. I've done both and found going around the PS pump easier, but my arms are long and like spaghetti...

The supercharged 3800 for some reason is sensitive to cheap wires. The coils are higher output than standard 3800 coils, perhaps that is why.