Yes, trak-lok is needed with a gear change. Why give it more torque multiplication, only to have it go up in smoke?
3.55's are the way to go on a car originally equipped with 3.08's. You can usually go up 2 "steps" in gear w/o requiring a tune. That will definitely save you some cash.
97 T-Bird LX 4.6 - 68k miles
94 Supra TT Auto - street/strip car
04 Crown Vic Police Interceptor - Brenda's car
95 T-Bird LX 4.6 - fully optioned, owned 15 years
96 Cougar XR-7 4.6- Brenda's car, owned 11 years
88 T-Bird 3.8 - first T-Bird, owned 5 years