I'd get an oil analysis done, before buying a 6.2L with that many miles. Keep in mind it's a light-duty motor with a light-duty design life (150,000). It's designed for power over longevity, and feeling sluggish could be low compression .
The lag is probably just the electronic throttle body. The earlier GM ones were slow to move, relative to a mechanical throttle body, and combined with somewhat invasive torque-management, can make it feel how you describe. And a high-mile one might need a good cleaning.
The AWD in that rig won't "steal" power, since it's still a mechanical coupling (clutch based engagement), and the clutches won't engage anyways unless it detects an difference between output shaft speeds.