Yes, the trak-lok is ford's limited slip diff, comparable to a posi in a GM. If you get one, you have to make sure it is for an IRS car, so one out of a mustang or crown vic in the junkyard won't work. Best bet is to find an 89-93 SC in the junkyard because they all got the trak-lok rear, or find someone on here selling just the diff and put it into your housing. You also may want to get a trak-lok rebuild kit since you are going to be in there anyway, and one out of a junkyard might be pretty worn out. If you are going as far as taking the diff all apart, you might as well get a TL in it.
Going from 3.08s to 3.73s you would also need either a j-mod or a tune. The reason is that there is a delay between when the computer tells the trans to shift, and when it actually does because the fluid needs to fill the accumulators and build up pressure. The computer knows what this delay is, and it knows how quickly the rpms rise in a given gear, so it can compensate. With the gears, the car will now accelerate quicker, but the computer won't know that, it will still tell it to shift at the same rpms, which means the rpms will now be higher than they should by the time the shift actually does happen, and you will hit the rev limiter on WOT shifts. The solution is either reprogram the ecm (with a tune), or make the trans shift quicker (with a j-mod).
-91 Cougar LS, coming soon, complete overhaul
with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
-"Your buddy Mike is INSANE!" -ClintD's dad