whole car shakes/jolts when turning to defrost

[2000Xplor3r]

so like the title says, i have a 2000 ford explorer 4x4 v6 4.0 sohc engine, when i turn the defrost on for the heat, you can hear the A/C kick on cuz my a/c does work, and the truck starts shaking in place and jolting like crazy as i turn the setting up higher... it is not even that serious in the other modes but can have the heat on front, feet, whatever setting until defrost and then it goes really bad, i have a few ideas about the compressor going bad? vacc line? blower motor? im looking for some advice as i dont know what else to look for or other symptoms or test, ive done thermostat, therm housing, all new green coolant, no blocked passages in hoses that ive noticed, but i was wondering also if a heater core is bad? i just dont get it

 

[rb142]

The only thing different there is the engagement of the AC compressor, so that has to be related. Since you say the AC works OK, I will assume the compressor is good. So the other part is with the compressor on, the engine load is higher and the rpm should kick up (or it will dip if the kick-up isn't working). That points to the long list of idle quality problems. I'd look for a vacuum leak first. IAC is another. So are MAF, low-speed misfire (spark problem, fuel pressure problem), etc. Too hard to diagnose without seeing it.