98 auto trans shifting problems???

[Dereks98]

So I have a 98 3.8L automatic transmission mustang that I bought about 2 years ago. Its never taken off very fast and other Stang owners that drove it told me the same thing. I know its only a V6 but it still should not go that slow. A friend told me it sounded like it wasn't shifting from first gear. Possibly taking off in second every time. I shifted manually after that and it goes swell. Taking off better than it ever has. Transmission was rebuilt at 172,000 and it had 179,00 when I got it. It has well over 189,000 since that's when the mileage stopped working. Is there anyway to fix this without tearing into something major??? If not then what would be the best solution.