Yeah it may be physically possible to get a solid axle to flex twice as far as this, but how do you think that's going to handle on the street? And at what cost would it be? (infact I don't think it'd handle all that well offroad either. It IS entirely possible to have TOO MUCH flex)
Like was said, extended radius arms, longer shocks and better springs all will go a very long way in getting more flex out of any suspension (and of course an improper steering linkage angle will need to be addressed too, which is pretty much guaranteed with most TTB suspension lift kits).