1999 F 350 &.3 diesel, 6 speed 2 wheel drive.
Real axle is a limited slip model however I sometimes get one wheel to spin but the other won't lock up to assist in traction.
Am I doing something wrong?
Thanks.
Eric

 

Whats the lube look like? Any higher concentration of suspended metals/friction materials?

You can test the' initial' breakaway toque at the axle flange, or manually check lock ops..

One wheel on a dry surface and the other wheel on ice, mud or snow.
Gradually open the throttle to obtain maximum traction prior to break away. The ability to move the truck demonstrates correct operation..

When starting with one wheel on an 'excessively' slippery surface, slight application of the parking brake may be needed to help energize the lock feature of the diff... Release the brake when traction is established. Use light throttle on starting to provide maximum traction.

If, within unequal traction conditions, both wheels slip, the limited slip rear axle has done all it can possibly do.
In extreme cases of differences in traction, the wheel with the least traction may spin after it has transferred as much torque as possible to the non-slipping wheel..

 

Haypower, thank you!
There was some gummy grey matter on the magnetic fill plug and the truck has 240K on it. Maybe it is time for a rebuild.