97 Explorer, Heat doesnt work at idle, only blows hot at highway speed >2000 RPM

First thing to check is coolant level when the engine is hot. If that is ok the check the heater hoses going to the heater core. They should both be hot. You could also have an open thermostat. Alex

Air pockets can cause this, possibly from recent cooling system repair work also. Besides what Alex said.

Google > 1997 Ford Explorer heater control valve ... View the resulting page.

Step by step diagnosis would be to inspect coolant level. If coolant is low, refill cooling system & confirm heater operation at this point. If heat is still weak after filling system test operating temperature of engine with a pyrometer. If engine temperature does not come up to 190 degrees (or manufacturer's specification) replace the thermostat & retest. If engine temperature is coming up to normal test both heater hose temperatures. They should be fairly close in temperature. If not , inspect for a heater control valve malfunctioning (most late model vehicles do not use a heater control valve). If valve tests good or no valve exists the heater core is restricted. Back flushing the heater core may be an option. Typically there will be engine over heating issues if the water pump impeller is failing.