Ok. I go to start my 1995 1/2 ton Chevy Silverado ext cab. runs good. as i start to back up, it makes a really loud rumbling sound that you can feel while sitting in the truck and you can also hear the rumbling sound while being outside the truck. It does this only in rev. It feels like to me that its trying to cut-out. Any help would be much appreciated.

If an engine is getting un-metered air, it will do that. Usually a small hole after the metering device. The engine torques in a different direction in reverse, so in your case the hole is being sealed in forward, and opened slightly in reverse.

where is the small hole after the metering device located near ??