Here is what is happening; engine idles smooth, seems to hit a little hard when going into gear, and has vibration when stopped in gear at idle, vibration strong enough to rattle the sideview mirrors a little, goes away as soon as I start moving. Probably been covered a hundred times before, and after a lot of searching, this is what I think I need to check first;
check for vacuum leaks
motor mounts
trans mount (this should be okay, was fine 200 miles ago when I installed SYE)
then? harmonic balancer, maybe flexplate?

Am I on the right track?

 

So the idle itself isn't rough?
​​​​​​Year?

Flexplate less likely. Mounts high on the list.
My vibration actually was more noticeable after replacing the engine mounts, I think because everything was more "solid"..

 

correct, idles smooth and runs good, does get poor fuel mileage...
1991 4.0litre

 

Have a look at all the mounts
XJs aren't vibration free, and it's normal to experience more vibration in gear at a stop than in neutral at a stop. It is otherwise smooth and running well.

 

Inspect the motor mounts and look where the exhaust pipe travels above the crossmember.

 

Do you have a helper that can hold the brake while you crawl around under the vehicle? Don't forget to chalk the wheels, due to the danger.

What transmission mount did you use? A rubber OEM style mount? Aftermarket polyurethane mount?

Since you put a new transmission mount in, you might have a strange angle going on, due to old sagging engine mounts & a strong new transmission mount.

 

Chock the wheels. Just in case.

 

Good info, I'll look into the mounts and exhaust.

thanks!