Need to know the location of the vacuum leak in order to answer this.
Check for vacuum leak
Might be a vacuum leak
I have a leak on my drive shaft left front 2000 cheve Blazer I would like to replace the whole unit
Start with looking for a vacuum leak.
Check the 96 Blazer for any vacuum leaks in the system. A vacuum leak can cause the air flow to only come out of the heat vent.
You failed to mention the model year but a large vacuum leak could do that.
Lean condition detected on banks 1 & 2 (left and right sides). Most commonly a dirty mass air flow sensor, or possible vacuum leak.
Sounds more like you have a large vacuum leak to me.
To repair a vacuum leak on a 1996 Chevy Blazer, start by inspecting all vacuum hoses for cracks, breaks, or loose connections; replace any damaged hoses. Use a smoke test or spray carburetor cleaner around the intake manifold and vacuum lines while the engine is running to help identify the leak. Once located, tighten or replace the faulty components. Finally, clear any trouble codes from the engine's computer and test drive to ensure the issue is resolved.
A vacuum leak could do that, also low fuel pressure or a dirty fuel filter.
Start with a visual, inspect all vacuum lines for problems.
change ur rear differential gasket or ur transfer case is leaking