95 explorer with 2004 engine and trans HELP

I got a 95 explorer xlt that the guy just had a 2004 4.0 v6 engine and transmission put in. It sounds great and runs pretty well with one exception. From 1st to 2nd gear you have to let off the gas to get it to shift. The guy i got the explorer from said there was a sensor that needed to be replaced in the back end. Does anyone know what sensor he is talking about. Your help is greatly appreciated.

Thanks