2001 burban revs up before accelerating

Can anyone tell me what might be causing the vehicle to rev up before it starts to accelerate? This happens on the freeway when I am trying to go uphill or speed up. There are no problems while cruising or city driving. Any ideas would be appreciated.

Depending on how much throttle you are tipping in, it sounds like the convertor is just unlocking and maybe the tranny is dropping out of overdrive to climb hills. My diesel burb does a lot of this.