Shutter/Vibration

When slowing down between 30 and 15 mph there is a quick shutter or vibration. It's not in the steering column (I've had that checked several times I was told the steering was surprisingly tight for a truck of that age). I feel it under the truck, pretty even across the bottom of the truck. I even feel it when I'min the passenger seat riding with the mechanic on a road test to try to get him to feel it. But if I bump the truck in neutral when slowing down it doesn't do it. It only last for about 1 to 3 seconds. I was told it was a broken motor support (which was broken cause I was informed by several repair guys that the drivers side support breaks quite often) so I got that repaired and it still does it but now it is very faint but I know its there and if I accelerate during the vibration it gets worse(kinda like a suttering truck not getting gas, so I got a tune up and fuel injectors checked and cleaned) but it's still there. Help, cause my mechanic is starting to think I'm off my rocker. and so am I cause nobody else feels it, they say the truck does that. but I've been driving Chevy's my entire adult life and I have never felt this before in any of those.