Is my 904 toast?

You can deduce a lot if you just "think". Normally a driveline/ engine vibration is so much higher in frequency than an axle/ wheel/ tire/ traction vibration, that you can often separate them immediately

Such things as bent driveshaft/ bad U joint/ loose pinion yoke or with stick, maybe a torn up clutch disk and so on. All those have a VERY high vibration frequency, more of a buzz than a "thump"

Have you pulled the driveshaft and checked the U joints? You can NOT check U joints by shaking the shaft in place. You MUST pull the shaft and carefully move them while feeling for dry/ rough movement. The would be the most basic of all failures.