I dunno guy.. I've had a few 3.8's and well into 200K miles.. never had an injector fail yet. I've seen one or two fail here or there on other folks cars..but not all 6 at one time. How about this.. I'll give you some questions and answers.. then before we spend any money... likely we'll have it fixed. Sound like a deal?

How many miles since you put the plugs in?
You are correct, in 99 the rear O2 doesn't affect fueling.