Slow to engage into gear (excluding converter drain back) is generally worn rear clutch discs, bad seals, bad reaction shaft seals, or VB problem. Not shifting into high is generally a front clutch issue or reaction shaft seals and since the front clutch is used for reverse it's affected too. The likelihood of both clutch packs or both sets of reaction shaft seals going at once is rare so I'd be inclined to think it has a VB problem such as a broken main pressure reg. spring or trash in the regulator. I'd do a pressure test and see what they look like.