I'm sure the factory designed the geometry so that the axle would bottom out on the bumpstop before the driveshaft butt-f***ed the tranny (see what I did there?)
Sure, there's very little slack left, less than what you found, but it still should not bottom out. The way the rear of these things are lowered (coils only) doesn't change the factory articulation of the suspension, so lowering it is basically like keeping the stock suspension in a "squashed" state at all times. Now, if you removed the bumpstop from the frame, the axle may be able to swing up further on harder compressions and then you may can do damage. Get some adjustable link bars or axle relocators. I'd go with the relocators if it were me. Besides allowing tons of adjustment for axle placement and pinion angle, the adjustable links are known to be noisy due to the type of bushings. If the pinion needs adjustment, maybe get relocators and just upper links?