I have 2001 blazer when driving when I load engine on incline ac changes 2 dash

more than likely you have a vacuum leak. check under the hood,should be a vacuum hose going through the firewall(most are smaller hard plastic hoses) they split/crack/dry rot over time.check all the vacuum hoses that you can see for leaks(some tee off and go to different places). start the engine,turn the air on(listen/feel around hoses for hissing sounds,air escaping). vacuum hose/repair kits/fittings/connectors in a kit at all parts stores.

Under hood - and approximately under the drivers side 'hood hinge' - actually tucked inside of the fender - you'll find the vacuum storage reservoir - they tend to develop leaks - that is the usual suspect. That is where testing should commence - IF unfamiliar with automobile repair - seek *hands on* help from qualified personnel.