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At 75 you should be seeing about 2100 - 2400 on the tach in fourth gear
(the 'overdrive' ratio) with the torque convertor clutch (TCC) locked
up.  Flooring the gas pedal should produce a smooth but 'positive'
downshift to third (or second with some rear gear ratios) and the tach
should smartly rise to >3000rpm. Depending on final ratio in your
individual car, you might see a downshift to second. The car should
accelerate upward from there with smooth upshift(s) as the tach exceeds
4K.  If there's surge or slip it's time to see a good auto-tranny guy
who works on Corvette automatic boxes.  The Corvette version of the
transmission has a couple of features (beefed up clutch plates and an
accumulator for '89 and subsequent C4s) that aren't in the truck and
Camaro/Firebird THMs).  There is some designed-in slippage during
shifts.  The transmission measures this slip -- when it becomes
excessive, servo pressures are automatically increased to correct it.
This yields "hard shifts" and the transmission should be looked at by
someone who knows Corvettes.  Rebuilt swapouts, at chain tranny shops,
aren't wise since you'll probably get a transmission without the
Corvette goodies.

Overdrive is a way of saying that fourth gear is not direct drive (a 1:1
ratio, as it was in earlier TurboHydramatic transmissions.)  The "OD"
fourth gear is less than 1:1 ratio, check the owner's manual--it will
probably show something like 0.78:1 etc.

As Charlie suggests, spend a few bucks on a shop manual. It has an
excellent troubleshooting chart on the auto-tranny with good
descriptions of normal operation, simple driving tests, and what to look
for in the way of typical problems.

Welcome to the world of C4s.  These are reliable, fun-to-drive cars with
good creature comforts and plenty of gadgetry.  Don't accept
snobbishness from C3 owners or "efete" behaviors from some C5 and C6
guys/gals--we all wave.  C2 owners are allowed to get snobby with
anyone!  (;-) ... PJ