Problems with 91 Chevy 2500

I have a 91 Chevy heavy 2500 with a 454. Four years ago we had the tranny re-built. Have had some engine issues that are mostly fixed now, or easily reparable. It's not shifting like I think it ought to. It has very little power in 3rd/overdrive and wants to downshift at the slightest hill. At the end of a 120 mile trip, it doesn't want to upshift once I get off the highway. I've had a suggestion that it may need a new vacuum modulator. Any other ideas? I think the tranny is going again & I would be better off driving it off into the river than trying to have it rebuilt again.