I think I know why. When you lifted the rear 4", you have raked your rear axle so far forward that in a slowing or braking scenario, the rear tires will actually rub the front of the rear cladding. This happens to me on the trails. I also used the 919s, but with no spacer, but I run a 34" tire, so there you go. I would take Tom's advice and lube everything, but those issues won't result in the grinding you hear. You can really check the rubbing, just rub some grease on the inside lip of the cladding, get on it, do some sudden slowing, and if you hear the rubbing, check and see if there is missing grease.
As for the solution, lower the rear end (take out spacer) or go hardcore and order some rear adjustable lower links from Indy4x and problem solved forever. You can adjust your rake, and make the vehicle ride better again.
Post up a pic of your VX from the side, I bet you only have an inch or so in front of the rear tire to the cladding...This is the best I could find of mine, but you can see that there is not a whole lot of room there in front of the rear tire.
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