WJL

All Trac-Loc rear ends (Mustang Rear Axles with the exception of IRS on Cobras) are Positraction and have clutch pacts in the carrier, some times referred to Limited Slip Differentials.

If it didn't have clutch's you would not be able to turn or it would be difficult to turn the car. If you have ever ran a 4X4 on a roadway surface and you feel that binding when turning, the reaction would be much the same.

When making a turning movement your cars outside wheel moves faster and travels a greater distance than the inside wheel the clutch packs allow this to happen without binding the axle.

WJL

Here is a pretty good explanation of how differentials work