Maybe this will help. My son's Dodge has a computer controlled transmission. I would expect your's does also. He had the same sudden downshift as you described.

What had happened was that the tranny computer had lost connection with the tranny. The computer then put the tranny into "limp home mode", i.e. downshifted and would not go back to a higher gear.

Here's the good part. A shop said it was a bad tranny, another said a bad tranny computer. My son checked around under the hood and found that there was a connector, which connected the tranny to the tranny computer, which was exposed to moisture dripping down between the hood and the cowling. He cleaned the connector contacts with a spray cleaner, greased them with a silicone grease, and reconnected. The transmission has worked fine for over a year!

A Warning. One shop checked the transmission and failed to reset the computer which the put the tranny back into limp home mode after a few miles.

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog