The 3/4 upshift is where problems show up in transmission hydraulics, because there has to be enough pressure and volume for both clutch packs to engage. A delayed shift could be caused by a damaged K2 accumulator spring or a clogged filter. So a good place to start is to drop the pan and replace both the filter and the spring, both pretty simple jobs. Do a full fluid change while you're at it. Some say Type F fluid is better for this condition than Dexron, not sure I'd go there. If that doesn't do it, it wouldn't hurt to try a magic additive. Look for something that claims to have viscosity improvers and seal expanders. When that doesn't work, you'll have to tackle the most likely cause, which is either a worn clutch or defective seal in the K2 pack. Repairing this requires tearing down the transmission.