Disturbing sound last night

[Christiank22]

So I was backing up last night and when I came to a stop to go into drive before I even switched gears I hear a loud noise kind of like a hard shift. I have also been noticing that whenever I come to a stop in drive the vehicle has a vibration and almost like a hard idle but the rpms aren't acting too crazy. I read that, that could be caused by having a bad O2 sensor after putting in my headers since I was getting some codes from the O2 sensors anyway so I'm gonna put those in when I get the second one. But now after hearing this sound I almost feel like it could be in the Tcase maybe or a tranny problem. I hope I gave enough information for someone to help.