I'm swapping rear ends in a ford xlt 150 lariat.
I have come across something I never seen before, an electronic plug that plugs into the rear end.
Would you know what it is for and do I need it?

 

That should be for your anti-lock brakes, if yours doesn't have them just tie it out of the way, it shouldn't be a problem. If it does you need to get one that has ABS or your brake lite and ABS lite will always be on.
It could also be some sort of speed sensor. You don't say what year it is.

 

It's a 1987

 

Anti-lock brake sensor.

 

I think it's the VSS (vehicle speed senosr) when unplugged the abs light will stay on, cruise control will not work, the odometer will not track mileage and the speedo will not work. At least on my truck thats what happened when a stick broke the wire while off wheelin Took me a year to figure it out.

 

On an 87 it's not the VSS. They only combined that function into the ABS sensor in later years.

 

On an 87 there is no abs, but it is the speed sensor for the speedo and the odometer.

-Jon

 

87's do have abs in the rear. I know this because my dad has an 87 2wd, 3 speed manual w/od and has rear anti-lock brakes. There is a speedometer cable that goes to the transmission. The cable was replaced in later years (don't know exactly when but I'm thinking 88 or so) they increased the functions of the abs sensor to detect the readings for the speedometer, odometer and also the abs.