[dsraven]

Some of the stock gm beehive snubbers, like the ones used on the rear of an S10, are hollow and bolt on through the hollow part. You could use something like that and incorporate a built in positive stop by using a longer bolt with a spacer under the bolt head. The snubber would absorb travel until the axle hit the bolt. The thickness or depth of the spacer would set the bolt height and therefore the upper limit of suspension travel.
Remember to do suspension travel mock ups with the axle at an angle to horizontal so you simulate one wheel going over a bump or whatever. That will check for tire rub on the box as well as axle hitting box, brake line rubbing issues, park brake cable issues, axle vent hose issues, shock absorber issues, etc.