well techniccally the last gear is the side gear inside the ring gear, and the transfer gears are actually upstream of the whole ring and pinion to begin with, but its all good.

background:

rwd transmission power flows through one basic shaft, more or less, in the front and out the back in a straight line. because fwd cars usually use transverse engine arrangements and transmission width has to be curtailed, what would be the tailshaft housing of a RWD transmission is replaced by a set of gears that translate the power downward to another shaft that drives the ring and pinion. that set of gears is the one being replaced in this mod.

luckily, since chrysler used the 604 on a wide variety cars with hugely varying weights and powerbands, they decided to vary the ratio of the transfer gear set to make cheap and easy gearing changes. the set with the best ratio for acceleration is what people are using and calling the 'final drive mod' tho theoretically you could use any of the different ratios and get an effective differential ratio of 3.17:1 all the way up to 4.17:1 or whatever it is the shortest gears come out to.

the reason people call it a final drive mod because it modifys the 'final drive ratio' of the transmission, which is also called by several different things but actually refers to the total ratio of all the gear sets from axle to tire in top gear. with the gears discussed here that comes out to something like 2.9:1 as opposed to stock which i believe is 2.69:1. you get the final drive ratio by taking the differential ratio and multiplying it by the top gear ratio, which in this case is .69:1, or the other way around by dividing. that name is accurate in that the gears change the final drive ratio, but inaccurate in that people really only do this mod to affect the performance of the 1st three gears, not 4th, lol.

i personally call it the 'transfer gear mod', because it makes more sense to me. hope that was in any way useful.

1993 Dodge Dynasty LE 3.3 Auto, 95k, Stock.
16.2@84mph