You must have been doing this on flat ground. Because if there were any grade there would be a load on the driveshaft and that would take up any slack.
Yes there is going to be backlash. Every set of gears, every splined shaft, they all add up. Any single one will be barely noticeable, but they will stack up into something noticeable.
And the Detroit locker has a lot of play in it. Between the teeth and the ramps it has to have play in it for the mechanism to work. That play is multiplied by the gear ratio. With a low set of gears, everything set up right, I've seen nearly a quarter turn of the driveshaft between reverse and drive (loaded each direction with an automatic).