The '87 has the exact same problem. It shifts way too early and is a complete dog around town because of this and because of a needed ALDA adjustment.

Now, the '86 is setup perfectly. You won't get the dreaded 3-4 shift until above 40 mph unless you are just barely opening the rack. If you step into it a bit, the 3-4 will be delayed to over 50 mph. And, if you mash it, the 3-4 won't occur until 60 mph or so.

All of this is related to the Bowden cable from the linkage down to the trans. The cable needs to be tightened a bit to delay the shifts. I need to do it on the '86 as well. But, I'm not fond of working in the cold.