After the J-mod, mine is also still sluggish in 3rd, even though the shift is crisp. It is the delay from when the shift occurs to when the TC lockup occurs. You are kind of in no man's land until it locks up!
Of course, this is low speed cruising only.
97 T-Bird LX 4.6 - 80k miles
94 Supra TT Auto - street/strip car
04 CVPI- Brenda's car - 76k miles
95 T-Bird LX 4.6 - fully optioned, owned 15 years, 220k miles
96 Cougar XR-7 4.6- Brenda's car, owned 11 years, 187k miles
88 T-Bird 3.8 - first T-Bird, owned 5 years, 206k miles