Reverse clutch band or other related component likely failed. I had a similar thing happen on mine around 281k....though it did not happen when using reverse. Happened after a 40 mile drive on the highway, heard a "swishing" noise....and reverse had become weak, but did not fail completely. I drive it another 400ish miles while reverse slowly died away and the forward shifting began to die and become erratic too. Never got stranded....swapped in a different trans, been fine since (over 25k so far).

I'd fix it or try to sell it to someone who will.

Here's the part I found in the pan (along with lots of clumped clutch powder) from my failed unit:

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