I guess the first thing I should start off with is to stress how important your seatbelt is and that you should not work on the assembly unless you are confident in your abilities. That said, it is a pretty simple mechanism that doesn't look like it would be easy to screw up. Besides, we aren't going to tamper with the locking mechanism.
First thing to do is to remove both of the plastic covers. Careful, the one that covers the retractor assembly is brittle and the tabs can easily break off. Here is a pic of that cover so you can see how the tabs secure it in place.
The upper plastic cover just pops off with a flat screwdriver.
Now remove the three bolts that hold the seatbelt using a 16mm socket. On one side of the retractor assembly is a white housing that very clearly states: "Caution Do Not Remove".
Remove the two phillips screws that secure the white housing to the metal assembly. Very carefully separate the white plastic portion from the metal assembly. Be careful not to open the white plastic housing until it has been separated from the metal assembly.
Now, with the "Caution Do Not Remove" side of the plastic assembly facing down on a table, unhook the cover and pop it off. This is what mine looked like:
You can see that the lower wheel is serving no purpose as the spring steel has come off of it. Oddly, this seatbelt did retract, it just needed a little help, which seems to be pretty common on these trucks. Take a large flathead screwdriver and rotate the upper wheel clockwise until the spring is tightly wound on the upper wheel and the end of the spring steel is on the opposite side. At this time it will spring down and engage the smaller wheel.
Now slowly turn the screwdriver counterclockwise to rewind the spring steel onto the lower wheel. Careful, that is the way it wants to wind up so don't let it get away.
Now put the plastic assembly back together. Next, tightly wind up your entire seatbelt onto the retractor assembly and put the plastic spring housing back on the seatbelt mechanism, but don't push it all the way down. With the plastic spring housing engaging the seatbelt assembly, rotate the plastic spring assembly clockwise three rotations, push it all of the way on and replace the two phillips screws.
It takes 14 complete revolutions to completely wind up the entire length of the seatbelt. It takes 19.5 complete revolutions to unwind the spring steel from the lower wheel. Thus, 14 revolutions plus the 3 additional winds of the plastic spring housing mean that when the belt is reinstalled in the truck and pulled out as far as it will go, there are still approximately 2 complete winds of the spring steel on the lower wheel.
After reinstalling the seatbelt assembly in the truck take hold of the seatbelt and slowly pull it all of the way out of the retractor, as far as it will go. Then slowly rewind the entire belt. This is both to verify that it is functioning properly and also to make sure that the spring steel has wound up tightly onto both wheels.
Also, a quick way to verify if the seatbelt locks up properly is to grab a hold of the belt, start pulling it out while at the same time smack the door pillar outside of the truck with an open hand. The mechanism should lock up.