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Can someone straighten me out on what was introduced when? Were all early 28s three speeds, then came the underdrive (dual high) and then the four speed? Either of which could be retrofitted to an older vehicle?

I'll take a go at it. Others please feel free to correct me with what I don't get right.
The 28's and early 29's came with the three speed transmission the same as the cars.
The dual high transmission was an option for the trucks with the high speed worm drive rear ends.
In mid to late 29 the worm drive rear end was changed to a bevel drive rear end and the transmission to a 4 speed.
The 4 speed transmission does not make for faster speeds but allows for 1st gear to be really low.
I may be wrong but the early 28 trucks only came with the high speed rear end and the low speed rear end was introduced not long after production started.
The flywheel housing used on the 4 speed is different from the 3 speed but the 4 speed transmission can be used on the early trucks and the three speed used on the later trucks. Flywheel housings will need to be changed accordingly.
I'm sure others will chime in with more precise dates.

I inserted my comments in Dave's comments:

The 28's and early 29's came with the three speed transmission the same as the cars.
The dual high transmission was an option for the trucks with the high speed worm drive rear ends.

NOTE - A Ford option for any AA with a 3-speed. If used behind a low speed axle - drive a stake in the ground to see if the truck is moving

In mid to late 29 the worm drive rear end was changed to a bevel drive rear end and the transmission to a 4 speed.

NOTE - The 4-speed started production in September 1929 with the worm drive axle. The bevel drive axle started production January 1930.

The 4 speed transmission does not make for faster speeds but allows for 1st gear to be really low.
I may be wrong but the early 28 trucks only came with the high speed rear end and the low speed rear end was introduced not long after production started.
The flywheel housing used on the 4 speed is different from the 3 speed but the 4 speed transmission can be used on the early trucks and the three speed used on the later trucks. Flywheel housings will need to be changed accordingly.

NOTE - And, other changes are needed to make conversions (i.e. 3 to 4 speed or 4 to 3 speed)

That is correct about driving a stake in the ground. My Dec. 28, has low speed worm with the 3 speed. And that is really slow.. But the good thing is that at a good idle speed in low and 2nd gear, it is perfect parade speed!! ( only silver lining in a low speed worm).