I'm swapping rear ends in a 1987 ford lariat xlt 150 4x4.
The rear end I took out has a electronic plug that plugs into the pumpkin and the one I'm putting in does not have this. Can anyone tell me what this plug is for and do I need it?
Thanks.

 

Speed sensor. If you have ABS or RABS you WILL need it. probably other uses as well, someone else will know more, particularly about your specific year...