the 5-speed is a NV3550. it was only available from 2000-04.
the 6-speed is a NSG370. it was only available from 05-06.
all 05-06 Rubicon's use the NSG370. the ratios are close together and 1st gear is a granny-type low. it's a damn good transmission thats more forgiving to gear ratios & larger tires.