I recently installed solid diff and trans mounts. About a week after. I was driving and when I started to decel I felt and heard a loud vibration that felt like the drive shaft or an axle. As soon as i put forward force on the drive train and start accelerating it goes away. Any thoughts? I checked and everything is tight.

 

No one?

 

Have you checked you U-joints? if you did both trans and diff, and have hooked it at all. You've probably taken already work out joints and used the last bit of life out of them. If so, time to get pliars, a BFH, and a punch, oh a dictionary of every curse word in the english language lol

 

solid mounts will give you more vibration in general

 

Research the forum, there are numerous threads dealing with drivetrain vibrations. The flywheel is sometimes at fault. Dual Mass Flywheels (DMF) do wear out.

For the OEM Luc manufactured flywheel for the Z, there is a nylon grommet separating the primary and secondary masses as well as springs that cushion vibrations. These can weaken or wear over time. Either will result in an out of balance flywheel which is most evident at 4100 and again at 1800rpm when gearing down, especially in 3rd gear.

Although a DMF can fail at any point, high mileage is a factor, as is aggressive clutch release.

A pretty good description, here:
http://www.honestjohn.co.uk/faq/dual-mass-flywheels/

If your DMF is at fault the pilot bearing and transmission input shaft bearings are in jeopardy, as are the rear seal and ultimately, crankshaft main bearings.

You might want to check this out?