I have a 2003 F150 SuperCrew 4X4. It's got street tires.
The ID plate says it has a limited slip differential (B6).
Yet I have never been able to get both rear tires to break loose.
Only the right rear will spin; even when its wet out.
I've also owned a Silverado & a Ram. Both of those would smoke the tires no problem. So how come my F150 is such a whimp?

 

it could be worn out, or need adjustment. It could be "pulling", but not at same speed as the other tire. Jack up the back end, turn one tire and see what the other is doing. You could pull rear cover, change oil, and inspect inside / clutch plate clearances.

 

Thank you for the reply.
If I jack up the back end and spin a tire, the other should turn as well, just in the opposite direction, right? And if so, then that would confirm it's limited slip?

 

no, if you have an OPEN rear end, the opposite tire will spin backward because the transmission is in park and driveshaft can not move..... if you have an LS unit, the tires should be locked together... since the drive shaft can not move, BOTH tires should be locked and will not rotate.

 

Got it.
Thank you.

 

Probably because their is too much amount of XL-3 FRICTION MODIFIER mixed in with the diff lube.

 

There is another way to test the relative lockup friction of the internal discs for wear.
Set up a test rotation of one wheel with the other on the ground.
Use a Torque wrench and socket on a lug nut.
Test the torque point required to cause the internal disc pack to slip.
It should be something over 40 pound feet on each wheel one side at a time.
Good luck.

 

So, you would apply the torque to the tire that's in the air, and anything under 40 lb ft would indicate discs are worn?

 

Yes.
It is a standard way to check the disc clutch operation.
The difference between a passenger car test and a truck test is the truck S spring tension is greater than a passenger car rear.
The slip point depends in the disc stackup wear thickness and surface condition, the spring tension and the fluid/ slip condition.
Good luck.