Recently had a freeze in coolant. Failed to test it before winter. Had to warm it up a couple times before it thawed so I could correct the coolant mixture.
Now, only get heat at driving rpms, not at idle...maybe thermostat stuck open?
Upper radiator hose is hard and I suspect a gasket problem.
This truck has been bulletproof for 240,000 miles...don't wish to break it now.

 

Yes, freezing will damage thermostat.

Rad cap should be rated at 14 or 16psi I believe, when warmed up the upper and lower rad hoses should be pretty hard.

When cold they shouldn't be, so if after cold start up the upper hose gets hard fast then yes you either have a cracked head or head gasket issue.
Overflow tank should also get full if this were the case, and of course, engine would overheat as coolant is forced out into overflow tank.

 

Engine temp stays in normal rang, but at idle, no heat. At road speed gets plenty of heat. I suspect thermostat stuck open?
Hoping not a gasket or crack in head...

 

Temp gauge should drop at idle if heater goes cold because of thermostat issue

I would do three things
Replace rad cap
Remove overflow hose at rad cap and blow in it, make sure it is clear
Disconnect highest heater hose at firewall, add coolant to rad until it is flowing out that hose, then reconnect hose, this purges any air in the system.