Need help! changed the heater core in my 97 F150 now the engine will not idle properly in when gear. thought it may be possible vacuum leak, but not finding any lines that looks bad with the naked eye

 

I am thinking vacuum also, except I'm thinking under the dash where the work was done. Where the vacuum line goes thru firewall, pinch it off, any improvement in idle?

 

I have exact same problem. I fixed 2 vacuum leaks under hood. One went to the back of throttle body, the other was the air intake manifold that was dry rotted and warped slightly. I still have the problem. It idles rough and when i get it to 45-50 ish miles per hour then it shudders. Sometimes it feels like it comes from engine other times it feels like it is coming form below the cab or toward the back. It all started from changing the heater core. I have spent $350 trying to replace different things hunting down the problem. So far I cant find anything wrong.

Gunna go try unhooking all the vacuum lines going under dash and plug them all to see if that helps.