200 Tahoe 5.3

Okay, here goes. When my 5.3 Tahoe gets to 40-50 mph and the RPMs are around 1500, if you press the accelerator it begans to stutter and hesitate, you can feel the vehicle shake from it. Once you drop down outof overdrive or get the RPMs up it smooths out. She had new plugs put in and a diagnostic ran but nothing came up. The mechanic did not put new wires on when he installed the plugs. I am fairly certain that it is not a transmission problem and figure it is most likely a ignition or spark control issue. Any ideas before I give a ton of money to GM?

have your throttle position sensor your crank sensor and your catylitic converter checked

try crawling under the truck, and hitting the cat with something blunt..and listen for a rattling noise.