My door code for the rear end is 86 which is 3.73 open on 2001 Ford ranger. The rear tag is also 3.73 with no L. When both rear wheels are off the ground and running in gear the rear end spins initialy but stops. When not running I van spin it initialy but it also stops. My truck is a 4x2, 5 speed, and 4cyl engine. The truck runs fine when wheels are on the ground. Please help.

 

When you free spin the wheel off the ground on both sides, to they spin opposite directions or the same?

 

If the truck is that old, isn't it possible someone upgraded the rear different before you got it? Just saying... I have a 99 with a 3.0, I has an 8.8 with a LSD, so I would assume anything is possible... my truck in theory was supposed to have the 7.5 with 3.73, I believe that since I have the "off road" package that it's beefier...

 

Older thread

If you jack up the rear tires and start engine and put it in to gear BOTH tires will spin, that only mean drive train is working, nothing else.

If you shut off the engine and put transmission in neutral, then spin one rear wheel the other will also spin.
If it spins in the opposite direction from your spin then you have an OPEN differential
If it spins in the same direction then you have Limited Slip