Apparently the info on the 1986 trans is true.
The info in the pic below is from www.autohausaz.com
I do not know what the shipping would be for 1 filter. However, if you order over $50 UPS ground shipping is free.
If changing the filter does not help I am not sure what the problem is. It could be perhaps that Kick Down Valve in the Trans stuck or malfunctioned and is not releasing; and, some aspects of your problem could be a B2 Piston Problem (depending on whos trouble shooting chart your read).
I have been a Mercedes owner for 2 years now and it is a Diesel. I am not sure on a Gas Engined Car how the Vacuum to the transmission is managed.
On the Diesel a Vacuum Pump creates the Vacuum. A valve on top of the Fuel Injection Pump controls the Vacuum to the Transmission Modulator (this relates to how harsh the shifts are; low vacuum=harsh shifts).
This deals with a Diesel but your trans is similar. But, I do not know if you have a Bowden Cable. From my notes:
TRANSMISSION STUFF Bodwen cable
Vacuum controls only the quality of shift transition.
The bowden cable on top of the valve cover controls shift timing.
Neither can cause a delay in engagement.
A stuck kick down switch can caused delayed shifts.
(I said valve but the switch must actuate something in the transmission.)
That is about all I can help with.