6 speed shiftable automatic on a 4 cylinder.

I'm no automatic transmission guy. Looking at the clutch & brake activation table, I would guess
that you might have issues with the K2 & K3 clutches.

Valve body issues (worn / dirty / plugged passages) or faulty shift solenoids can cause shifting
issues.

Any trouble codes stored in the TCM? Any aggressive driving in the past? Does the Transmission
fluid smell burnt?

A good transmission shop should be able to do pressure tests & diagnosis to find the problem.

If you have already gone that route and they said you need a tranny, then I'm sure their
diagnosis was correct.

Thomas