1996 Chev Transmission problem

1996 Chevrolet 4x4 suburban with 230k mi.

About 2 years ago the reverse went out and I parked the car for about 8 months. Used it once and realize forward gears work fine, but RPM needs to get over 3000 to auto shift up to next gear. Never makes it into overdrive. Have been driving about 1.5 years (only forward) city and highway miles up to about 70 mph.

First mechanic I asked said without inpection that the transmission would need to be rebuilt or replaced. Is that the only or most likly scenario?

Thank You.