well before my swap even got off the ground my 4speed maunal lost it looking for a trans for my 2000 3800sc when it list two trans codes for the 2000 pontiac grand prix whats the difference in the two?
if u look up car-part 2000 grand prix 3800sc it list two different trans

no difference, the 03+ had a different valvebody CONFIGURATION, the A and the B transmissions were the same they just had different valvebody STYLES.

The other thing that might or might not be consistent is the xb's used a quick connect transmission line fitting, and the xa used a flare type fitting for the cooler lines. They changed it over about the time they put the different valve bodies in.

i talkd to the salvage yard i got my 3800 from and they list the trans that came off the motor a xbb. they also want 600 for the xbb trans! they list a xab for 350 with less mileage?
will the xab work with my harness and ecm?

Again, there is no difference, the only question you have to ask yourself is "is it 03+?".