Ncobalt

2000 Tahoe with a 5.3, 4l60e. I bought a Tahoe with a bad rear differential, swapped it out. Both driveshafts disconnected. My question is, when I put into Drive, the transmission takes off on its own. Like someone is press the gas pedal. The speedometer jumps to 50mph then idles down to 20mph?. I am leaning towards a bad torque converter. I never seen this problem before. I need help if anyone heard of something like this