Subject: 1987 Dodge Raider vibration
Question:

1987 Dodge raider (mitsubishi pajero/montero)2.6L 4x4 manual trans.This vehicle shows vibration that can be felt inside the car.
This happen at 3000 rpm's on the road .
Also the symptom is present at the shop with out the belt connected.
Answer: Ignacio,
There are a number of items that can cause engine vibration. When you say that it happens at the shop without the belt connected, I'm not sure what belt you're referring to. Common causes of vehicle vibrations (not including tire/wheel imbalance) can include damaged harmonic balancer, broken engine mount, improper ignition timing, or possibly damaged internal engine component (piston, rod, etc.). Be sure your mechanic checks those areas.
Good luck!