A strong vibration is felt around 1500 rpm and continues on with varying intensity throughout the rev range. Tested while sitting in park.

It is not a misfire or a cylinder with low compression. Just replaced plugs, wires, cap and rotor and it passed CA emissions test and is running extremely clean. It feels like a rotational mass is out of balance. I've replaced the water pump and fan clutch recently. The balancer is still in its correct position. The vibration was present before those parts were replaced.

It doesn't really ever go away when either cold or warm but is less noticeable when driving.

It feels like an out of balance torque converter.

Could it be motor mounts?

Any advice would be appreciated.

Vehicle is a 1995 SWB with the original 3.9. 160K miles.

 

I would check the motor mounts first.

 

Ok. I ordered replacements and will install them. Looks like the ones in there are old and show small cracks in the rubber.

 

Sounds like motor mounts, but check the belt tensioner pulley for play/runout and be sure it's spinning freely.
Check the torque on the tensioner mounting bolt. Then also check the range of motion/play in the serpentine belt tensioner ( ERR3440 ). With a 15mm wrench/socket you should be able to move it without a struggle. If the spring is toast it can cause a vibration.
I went through this once with a '95 RRC and it ended up being a bad bearing in the pulley. It drove me mental. Good Luck!

 

Thanks OverRover. When I replaced the water pump and fan clutch I looked closely at the tensioner. It had a strong spring and spun quietly.