The Hammer works!! It's inside the cast housing the clutch collar. I can't access it without removing the chuck, but it actually works.
I had made an entire video decicated to cleaning this thing out, but I realised upon editing that it was unbelievably boring to watch and wouldn't add anything to the journey of fixing this drill.
In that disassembly video, I had established that the three sets of Planet Gears had different sizes for the different ratios required in both the reduction set of gears and the two sets of transmission gears. Counting the teeth from each set of Planet Gears helped to reassemble this, and having dissassembled several gearboxes, this characteristic seems fairly common. Most common cordless drills have two speeds and thus only two sets of Sun & Planet gears - the reduction set coming from the motor, and the set used to switch speeds. The DeWalt DC988 is unique in that regard.
Although the Sun & Planet gears in transmission systems like this seem to have more than just the Sun and Planet gears. I call the gear that is used as the base for the pins of one set of Planet Gears and the next Sun Gear in the lineup a Moon Gear (because why not), and the gear switching from the Moon Gear and Planet Gears is the Ring Gear. There are three Ring Gears in this particular gearbox - two of which are used in the transmission and the other is also used as the slip mechanism, which appears to be broken. If I had access to a milling machine, I'd make a new Slip Ring out of HSS, but because doing that is cost-prohibitive, and this drill is only used to drill through aluminium and mild steel to hang things for Powder Coating, I decided it wasn't worth the trouble.
Curiously enough, the slip mechanism is on the motor side of this gearbox, whereas more common drills, like the Ryobi R18DDP, have the slip mechanism around the Chuck Lock. The Chuck Lock, as the name explains, locks the chuck out of rotating the gearbox as you're turning the collet to either remove or replace a bit. This is a feature unique to drills with a keyless chuck.
