Have you tried looking into electrical problems?

This may or may not help, but...

My transmission was going into 4th at 30 mph. This is inline with what the FSM calls for, if I recall correctly.

The bigger problem was that I didn't have a passing gear.

After a multitude of linkage adjustments, I took it into a local indie who tracked the problem to a faulty kickdown solenoid.

Now that it's been replaced, the upshift points under hard acceleration have moved upwards (45 mph before it goes into 4th) and the kickdown is back. There obvious a role for the soleniod (or other electrical component) that governs how long to hold the lower gear.

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1974 240D "Boldie" 170K.- New timing chain/freshly rebuilt IP/replaced valve seals/injectors/upgraded stereo/new Bilsteins with Yokohamas/fresh paint and rocker panels plus lots of welds.