UPDATE - - - Most possible cause is listed by ' JWKO ' and a PM by ' guitodd ' .....
Although I have not comfired this (my problem went away) they both seem to be very knowledgable and have done research, and both say this is a CPS shaft (cam position sensor shaft) problem ....
Please read above for all deatails, and here is the PM from guitodd ...
Here is something you might want to take a look at. I had a mechanic friend take a look at the truck. He's a diesel mechanic.. but can be bought cheap with beer. The squeaking sound was intermittent for a long time and eventually got worse and was constant.
Even though it REALLY sounds like it's coming from the bell housing area.. it ended up being the cam position sensor that is mounted in the distributor hole at the back of the engine. (since these 3.0's don't use a dist.) It's a plastic sensor that mounts with two bolt-head screws and a clip on/off wiring harness and connects just like a rotor cap would to a distributor shaft. They are $20 and all the AP stores around my house carry them.
He was able to find the source with a stethoscope since the noise seemed to be amplified at the bell housing.
Not sure if we share the same problem as it seems yours went away... but if it does it again.. press your finger on the top of this sensor and you will fell the grinding going on inside if this is the issue.
A nice cheap fix..