This was just uploaded to YouTube today by "wiltatkansas," a Spaniard with an astounding collection of vintage NBA/ABA videos (seriously, go through the whole list sometime, it's impressive).

It's Game 2 of the 1978 WCSF, Blazers vs. Sonics, 1st and 2nd quarter action. At 00:50 the Blazers are beat down the court for a fastbreak bucket, and a limping Bill Walton calls timeout in order to take himself out of the game. I wasn't positive because the video skips ahead a bit after the timeout, but a quick check of David Halberstam's "The Breaks of the Game" confirms what I thought: these are Bill Walton's final oncourt moments as a Blazer. The would-be dynasty, in effect, ended right then and there.