yes you should have a undercoat what ever it means
Check your u joints on the drive shaft or the transmission mount..
drive line vibration could mean one of several things, none of them will end well if you do not fix the problem.1-the drive shaft could be out of balance due to bottoming out or somehow getting damaged.2-the swing bearing cushion that is in the middle of the two piece drive shaft could be torn or damaged.3-the gears in the rear differential could be damaged or you could need to add fluid.I hope this helps, if not i would let a mechanic look at it, but do not drive it for long with drive line vibration, you could seriously damage parts of your z.
Worn/loose u-joints are very common.
stockbridge
check that the pinion nut on the differential is not slightly loose. have drive shaft balanced. check transmission and engine mounts
Drive line vibration, it is possible that your drive shaft/axle are now out of alignment which would cause the vibration. I have the same problem with a 2 inch lift in a 98 Cherokee. I am going to try 2 inch shims (there angled i.e. little triangles) that fit on top of the axle between it and the leaf spring. Hopefully it will fix the problem.
it could be the frame of the car doing that my 2003 p71 crown Victoria does the same thing whan i go 135 on the speed way. check if your car has a recall on it. also it could be the axle or bareings. Aside from checking all the bearings, have you checked the drive shaft for balance? If not, have it done, and at the same time check the U-joints.
check cv axles.
so they can line it up after it it is took out
Should be about half way at the "belt-line"
A line drive is a ball hit really hard and fast in a straight line.
Reduce the closing rate on the valve.