Forgive the guesswork, but I'll try to confirm roughly.
Let me assume you're talking about an airplane flying fairly high and that the dropped Hummer reaches terminal velocity before it hits the ground. I think that is roughly about 200 mph (90 meters per second) for relatively dense objects like, say, Hummers.
Let us further assume that the tires and wheels collapse,the axle impacts the ground, and it penetrates some distance into the ground - lets say three feet (these are all just guesses - nothing scientific about them). Thus it travels about 4 feet after the tires impact the ground surface. Let us further assume that the deceleration of the axle is basically uniform (it would be parabolic, but we're just looking for a rough answer here). Thus the axle would travel the 4 vertical feet at an average of 100 mph (45 mps), which would take 0.03 seconds to stop. That would be an accelleration of about 1,500 meters per second squared. 1G is 9.8 m/s^2. Thus, the axle would be subject to about 153Gs - more if it stopped shallower and less if it penetrated deeper.
It would actually be greater because the deceleration would be concentrated more at the end of the stop than when it first hits.