Lifted 4" noise when decelerating from 50

[threeonthetree]

Just got the truck back on the road. I have a 1979 blazer frame with a cab and clip on it. Sm465 trans and a np208 transfer. I had the rear drive shaft shortened from my short box. New u joints. All fluids topped off.

My problem is when I am starting out it sounds great up to speed it sounds food as long as I'm on the gas.
If I put it in neutral going 45 it makes a bad grinding noise put the clutch in it still makes it and if I rev up the engine it doesn't change as I slow down it seems to go away. Or if I put it back in gear.

I can't tell were it's coming from. I think my driveshaft is a little short could that be the it?

Please let me know your idea on it.

To short?

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