Around 18" I would guess. This website:
http://www.expeditionswest.com/equip...iculation.html
claims 18.5" rear axle travel for a 2nd gen Trooper with OME shocks, which are a little longer than stock shocks.
With the N85 rear shocks, which are over 3" longer than the stock size, you can do even more:
Of course then you run into other issues, such as your coil springs falling out...