Your symptoms sound like the problem I'm currently experiencing with my 544 (Dana 27) and once experienced with the '70 145.

I was also getting a snapping noise and whinning when going downhill off the gas in gear.

I pulled the cover and found lots of slop within the spider gears. I could also turn the pinion about 15 degrees before the an axle would turn. The slop will eventually wear the spider gears axle and carrier that holds the spider gears. Sooner or later, the drift pin holding the spider gear axle to the spider gear carrier will fatigue and break letting the spider gear axle bounce against the pinion. Fortunately the one in the 544 hasn't broke yet, but it is keeping me from driving the car.

I did have this same thing happen on the 145 and that one eventually did break rendering the car undriveable.

I'm still trying to find someone who is willing to take on the repair. Most shops where I've inquired don't want to have anything to do with a case spreader. But someone must still repair these. The search goes on, although I'd like to try the repair on my own.

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Tom - '60 544, '68 220S, '70 145S, '86 745T, '06 Mazda MPV