Hi,

I have the same '86 Ford Ranger diesel and here is what I would check first:

1. Turn off the engine and start checking all of the accessories attached to the drive belt. Wiggle them to see if anything has come loose. If you have A/C, check the compressor for being loose and also test the A/C clutch.

2. Inspect the drive belt(s) to see if there is any unusual wear. If it is worn in an area, inspect all pullies and tensioners.

3. Remove the drive belts and check bearings in all pullies to make sure they spin freely and without excessive noise. Reinstall belt or replace if worn. Make sure that you tension the belt well.

4. The 4D55 has a timing belt and a balance shaft belt and they both have their own tensioners. If the balance shaft belt slipped off track or broke, it usually takes out the timing belt and the engine stops when the valves bend on the top of the pistons. I guess it's possible that somehow the balance shaft belt is either off track or missing. This would cause excessive vibration that is normally dampened by the balance shaft driven by that belt.

If you can't find anything loose on the engine and the drive belt looks good, I personally would recommend tearing down the front to make sure those belts and tensioners are ok. It will be worth every bit of the effort if you end up preventing catastrophic engine damage from a broken timing belt. (this is an interference engine)

Good luck. Take good care of that truck and it will take good care of you. Those things will run and run and run and..

About the only things that will kill it are running it w/o oil in the sump or w/o coolant. (or letting the engine belts go too long between changes)

Powell