With no or an erratic VSS signal the trans function is directly related. When the pcm doesnt know how fast you are driving then it has no idea when to make the shifts or command lockup. A scan tool is the only way you are going to know what the signal is doing going to the pcm and that it is reading correct speed or not. If the differential is failing then a lot of metal can accumulate on the sensor and cause erratic readings. Also in a few rare cases it has been noted that the reluctor ring on the diff can be loose and cause this but I havent personally ran into this. Get ahold of a scan tool if you dont have one and watch the VSS to make sure you are getting input.
