ALWAYS jack the car up and count driveshaft revolutions compared to 1tire revolution to determine your rear axle ratio. Unless you are absolutely certain of the entire history of the car since it was new, examining axle tags or VIN numbers won't always give you the right answer. MANY of these cars have had gears swapped, or complete axles swapped over the years.
Once you've determined what you've got (and as others have said - if they haven't been changed, they're likely to be 3.08's or 2.73's) then decide on your replacements. If you've got 2.73's in there now - you'll notice a big difference with either 3.55's or 3.73's. There's only 5% difference in engine speed between the two - you can almost toss a coin to decide. If you have 3.08's - I'd go with 3.73's just because they offer a bit more of a change from where you're already at, compared to going with 3.55's.
As JT says, Traction Lok is Ford's trade name for it's limited slip differential. This device allows power to be divided between both rear tires for more traction, while still allowing the inside tire to turn more slowly than the outside tire when you turn the car. There are lots of ways to provide this function - the TracLok uses springs/clutches to do it -- and those units do wear out. While changing your gears, it's the ideal time to freshen up the springs/clutches in the TrakLok. Those units also require a special anti-friction additive for the rear end lube - be sure the additive is in the dope that's used, or gets added separately. Otherwise that unit will make lots of shuddering/chattering noises especially when turning sharp corners at low speeds. For cars without TrakLok, or when the unit has failed, burnouts result in only one tire (right rear) providing the power. Which cuts your traction in half -- not what you need when you've multiplied your applied torque by about 30% with a 2.73 to 3.73 gear swap.