I have an LBZ with 40K on it with no towing or racing on it. How can you tell if the trans or motor mounts are going bad? I have looked at them and they look fine and bolts seem tight. Just have a bad rocking/vibration motion at idle.

Thanks

Why not take it back to he dealer?

i have owned the truck since it was new. Its been back over and over

Have someone start it while you watch the engine. If it rocks alot then you need mounts.
Also you may want to have them power brake it, as just reving it won't produce much power. But same idea it should't move to much. (It will move they are rubber mounts to my knowledge)

Looking at them they shouldn't have any cracks.
You can also try to lift the engine and trans with a jack. I wouldn't go far, but put a board under the oil pan and see if the engine will (lift) off the frame. Shouldn't go much. Same for the trans.