You must have no vacuum or minimal vacuum getting to your modulator. Worst case, somehow your valve body is not responding to the correct vacuum signal - but - since you have a rebuilt tranny I would not sustpect this first.

From what I know, the car shifts 'well' under heavy load because if the vacuum system was working properly, the modulator would see no vacuum anyway - the VCV (lastic thingy on top of IP) would vent it all out. That's what the system is meant to do.

I would start with verifying that your vacuum modulator can hold vacuum. Maybe you have already done this. What have you done?>

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car