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You can get various forms of traction assist for your rear axle without replacing the whole thing, but that would require rebuilding and setting up the differential. Available choices in the market are clutch type limited slip, including Ford's own, Torsen or helical gear type, and Detroit Locker type. You can also get the air locker type that activates manually with air pressure.
Finding a whole axle with the ratio you want and a limited slip differential would be easier to install. Those will probably be Ford's stock clutch type differentials, but they're cheap and easy to rebuild yourself.
"Posi-Traction" is a trademark of Eaton, which built units mainly for GM cars. Ford's trademark is "Traction-Lock".
1959 thru today: If the AXLE code stamped on the Rating/Warranty Plate (1959/79) or printed on the Certification Label (1980 thru today) begins w/a letter: Limited Slip.
But, if the AXLE code begins with a number: No Limited Slip.
FTE member and pal of mine lives in North Pole, bought a new 2003 F250 with 2WD. Salesman said you don't want 2WD, but he did. He put over 200,000 miles on it and never had a problem.