My understanding of the Fuel Trims is as follows:
The ST and LT trims are additive to the injector duty cycle. So, if you have LT of +8 and ST of +2, you are going to get 10% more injector open time. (This I'm NOT sure of, but I think it's how it works).
The LT trim is changed when ST trim stays at an elevated (away from zero absolute value) value for a specified amount of time (not sure how long) at the same engine conditions (MAF, Load, TPS, etc.). The goal of the LT trim is to keep the ST trim near 0 absolute (I think +/- 3 or so).
As far as one bank being much different from the other, my guess is that it could be clogged fuel injectors or intakes, or a dying O2 sensor. But, these are only guesses and I don't have my Service CD handy. I'll try to give you a more complete answer later. Can you give me the exact code you read?
The lean codes fire when LT + ST trims > +30% (i.e. the PCM is at it's wits end trying to add fuel to stay stoich in closed loop). Check the "Freeze Frame" for what was going on exactly when it fired.
