I have a 1993 Cherokee and when I turn my heater on my voltage drops significantly. I have replaced the battery and alternator about 2 weeks ago. I'm not really sure what's making the voltage drop so much. I'm new to Cherokees but I'm mechanically inclined

 

Why did u replace the battery & alt? What where your readings on the battery at rest and with truck idling? Also does your heater fan motor squeeel?

 

Very good question!
I would start by checking all of the grounding points. All theat need to be cleaned would be. Both battery terminals, battery to fender, oil dipstick, rear of head to the fire wall, 2 on the coil and the mounting area between the starter and the engine block. Also make sure the the terminals on the starter and alternator are clean and tight.