If my 86 4.9L had the 3.55 gears, I would have never owned
my 90 with the 5.9L.  With the change in speed limits, the
4.9 was simply working full time at 70 mph and had a top end
in 5th gear of 82 mph if I had a couple of miles to wind it
up.  It was considerable better in 4th gear.  It was in
serious need of more gear.  I believe the 3.55 would have
bee pretty close to what was needed for it with 235 tires.
If you have 31 inchers as many folks installed on them, a
3.73 would be more like it.  You will need to change the
speedometer drive gear in the transmission which will take
every bit of 15 minutes once you get hands on the correct
one.  A knowledgeable parts man or even a few web sites can
help you determine the correct part number or tooth count
for an accurate speedometer.  IIRC, the drives usually cost
around $15US.  Is this a worthwhile swap?  If you are
looking for better performance, it it one of the best
upgrades for the cost if you like your truck.  Expect to
spend around $500 parts and labor to change the gears.  A
far better route to me is to look around for a complete
salvage yard axle from another truck.  Many of the V8's had
the 3.55 gears from the factory and you can simply bolt in
the complete axle in a couple of hours.

Lugnut

The speedo cable goes into the trans on the drivers side. Remove the
retaining bolt and pull the cable out. On the end of the cable is a
plastic gear held by a C-Clip. Remove the Clip, swap the gear.

The factory gear might have 19 teeth, you can buy a new gear with 20,
21, 22 teeth. Each additional tooth is something like a 4 to 6 mph
decrease on the speedo reading. More teeth slows down the speedo, less
teeth increase the speedo reading.

I had to swap my speedo gear to adjust for a different tire size.

I don't know anything about swap the gears in the differential.

You could put on several sizes smaller tires on the truck. The smaller
tires would require the engine to turn more rpms to achieve the same
previous ground speed. This would simulate having a lower gear ratio,
and let you experience what the difference would be with a lower gear
ratio.

You don't have to change anything in the tranny, with the exception of a
very small gear that is used to calibrate the speedo -- and current
technology may make that unnecessary. The speedo gear, if you need one, is
easy to change. If you can change the ring and pinion, the speedo gear will
not be a hurdle at all.

What you need to change is the Ring and Pinion gears (yes, there are two --  
one each).

The gear ratio is determined by dividing the number of teeth on the ring
gear by the number of teeth on the pinion gear. (41 / 10 = 4.10, for
example).

I don't know the specifics of the gear sets that Ford uses, but the more
common ratios used in the ranges you suggested are 3.07 and 3.73. The
numbers aren't as important as the means of arriving at them.

Normally, one would change the gear set in response to getting oversized
tires. Larger tires will alter the gear ratios and move the torque and
horsepower curves to places that make torque and HP distant strangers. One
applies a new gear set to bring the torque and HP curves back to where they
are useful once again. With larger tires and the same gear set, you will
notice that your truck has no power off the line, slows going up a grade on
the freeway, and you frequently must downshift to the next lower gear --  
some report that they downshift so often, they stopped shifting to 5th gear
all together.