I don't know what you mean by booster cover, but if they are replacing an axle shaft you have most likely been bitten by the same problem described in this thread:
Some GCs with build dates from Feb and March appear to have gotten a bad batch of axles from the supplier. Seems the boot clamps weren't tightened down, allowing the grease to escape and get flung about. If some of that landed on the exhaust, or your turbo, it would probably smell pretty bad.
It also appears to be limited to GCs with the 2-speed transfer case. Those axles are a different part than the ones used for non-2-speeds.