GMC C2500 Engine Revs Up

Looking To Purchase This Truck It Has The 8100 Engine. When In Park The Engine Won't Rev Past 3000 Rpm But Driving It Works Ok. Any Ideas?

My Suburban Runs Good Up To 2400 Rpm In D (overdrive) But Then After 2400 The Tac Intermittently Jumps Up 50 Rpm And You Feel A Slight Lag...