2000 GMC Sierra Idling at 3,000 RPM

Transmission problem
2000 GMC Sierra Four Wheel Drive Automatic 155, 00 miles

I just had the serpentine belt and a new tensioner put on. The next day the truck began to accelerate without me applying pressure to the gas. With the brake all the way pushed down, the slowest the truck would go was 5 mph. I put it in neutral, it was still revving so I put it in park and turned it off. After a few minutes it was fine, but once I began driving again the same thing happened. When I left it in park and turned on the engine would run at 3,000 rpm. I tried pulling the gas pedal, but I figure there must be something wrong under the hood.
Sorry for the long explanation and any help is greatly appreciated.