I believe your tranny should accept the K-1 valve body retrofit.
The K-1 valve body handles the 2-3 upshift sequence. Apparently, the springs get weak over time preventing the K-1 valve from pressurizing quickly. This translates to the 2nd-3rd shift point "flaring" or slipping prior to engaging.
The K-1 kit supposedly solves this by providing stiffer springs to replace the OEM K-1 springs.
I did this kit retrofit about a month ago (93 300E), and not only did I not get rid of the 2-3 shift flare, but I now also have a 3-4 shift flare as well.
I believe I will let a tranny expert diagnose my valve body issues later this month...