Drop the trans pan and you will see a nut on the shift shaft. Make sure it's tight and there is also a clip on the shaft so make sure that is in place.
You might also want to make sure the shift cable is adjusted correctly. You didn't mention what kind of shifter you were using but for most aftermarket shifters you will want to pull the shifter into the low position and ratchet the trans selector into low and adjust it from there.
You might also want to make sure the shift cable is adjusted correctly. You didn't mention what kind of shifter you were using but for most aftermarket shifters you will want to pull the shifter into the low position and ratchet the trans selector into low and adjust it from there.