The entire axle housing, or just the Trac-Loc?
The rear axle as a whole will fit in a Pinto, but it will need to have the mounting pads changed from the coil sprung setup of the Mustang to the leaf spring setup of the Pinto. There may also be an issue with the width, but I am not completely sure.
If you are asking if the 7.5" gearset will fit in the 6.75" standard rear of a Pinto, no. These Salisbury type rears are similar in appearance, but share few if any mechanical parts.