I got a rebuilt 5.7 engine put in my 96 suburban 2yrs ago as my original 5.7 which i thought died on me at 200,000miles(long story but the mechanic was wrong about needing a new engine) and at first the engine ran fine at idle, i had constant oil leaks that the mechanic shop did not want to fix and after the last trip to the mechanic shop(my last time to take it back to where i got the engine) i got the suburban back and since then has a rough idle at 700rpms, you can feel it throughout the vehicle that it is idling rough and you can feel the vibration. Ive taken it to the dealership to get it looked at and found out it had autolite spark plugs in it so i changed those out with the recommended delco plugs. The spark plug wiring is the premium plug wires from napa that have the lifetime warranty. The distributor cap was replaced as well. What I think it is, is a possible vacuum leak somewhere, for when i have it in gear the idle drops back down to the default idle of 550? When it is in park or neutral it goes straight back to idling rough at 700rpm. The dealership could not figure out what was causing the rough idle as they could not find anything wrong. the rough idle started right after the mechanic at the shop i got the engine thought my intake manifold was leaking but then found it was leaking elsewhere, i think he might have forgotten or messed up a vacuum seal somewhere possibly? Any help would be greatly appreciated as I am clueless on this other than a possible vacuum leak but i dont know where to start for that and how to check intake manifold. The mechanic at the shop did not replace any of the belt pulleys and the belt tensioner and idle pulley have never been replaced. All of the pulleys are original parts.