Vibration at 1100 rpm
 

This is my first post here so bear with me. Got my Mustang about ten months ago. Ran and drove perfectly when i bought it. After about 4months it developed this problem. Discovered that the diff was low on oil. Filled it. problem went away (mostly). Now it's back. In any gear, at any speed (especially going slower) there is a heavy vibration at exactly 1100 rpms. I can feel it in the seat (my butt), in the floorboard (the gas pedal), and in the steering wheel. if i accelerate faster it moves through the rpms faster and the vibration is not nearly as noticeable.

Do you get the same vibration at 1100rpm in neutral? If so, the rubber sandwich in the harmonic balancer may have perished causing the outer part of the balancer to separate.

I do not. It revs fine to 2000k and back down to idle. Only happens in gear.

OK. Then it's likely that the vibration is coming from the front end of the drivetrain e.g. flywheel, clutch. Anything aft of the tranny would cause vibes that are related to road speed rather than engine rpm.

not really any other complaints except clunky shifts but as far as i have learned that's normal. have inspected the trans mount and checked all the bolts. Everything seems fine. Is there a particular part of the mount plate i should pay attention to when inspecting it? 2009 GTCS. 123,000 miles.

From the front is what i am thinking as well. Have you ever heard of wheel hubs going bad and causing this kind of thing? They are becoming suspect and my plans for today include a complete and thorough inspection of them. It also could be the clutch/flywheel/throwout bearing/pilot bearing/input shaft bearing. I'm pretty sure all that jazz is original/never replaced.

Update
 

I put the rear on jackstands and ran it through idle to 1500k rpms. the vibration is still there but very faint. Almost like a hint of what i experience when i'm driving down the road. Also, the rear wheels remain slightly driven when the car is in neutral. That last part was a surprise to me as i thought being out of gear completely de-coupled the engine from the transmission.

You're right about the latter so it looks like the clutch isn't releasing completely. If you still have the original clutch from when it was new, it's probably shot and that would also account for the clunky shifts.
Worn wheel hub bearings can cause vibrations but they'd be road-speed related and worse when making turns. They aren't related to engine rpm.

Vibration at 1100 rpm
 

I've been reading all morning about parasitic movement in drivetrains. I think the wheels spinning slowly might be ok. Seems like a problem if they spin half or more of idle. It seems to be caused by cold oil/grease which it was 25F here last night. The problem is definitely trans area or back. The problem could be in the clutch area for sure. There have been times that i thought it could be slipping but then when i start out on an incline it never feels like it's slipping. I've burned through a few clutches in 4x4s so i know what it feels like when they start to let go and mine feels pretty solid.