My van has developed a few strange transmission issues that I am hoping someone can shed some light on.

First off, what is the transmission doing when you feel what seems like a small shift within a given gear. Is this the torque converter locking up? In other words, if the vehicle has just shifted into 3rd, and I put my foot into the gas (but not enough to make it down shift), about half way through the power band, it feels like it makes a small shift and it pulls a bit harder than it was before.

Part of why I am asking about this is because the van has been making a loud high-pitched squeaking noise that seems like it is coming from the transmission. The noise is similar to a belt squeaking, but the pattern is completely different. It does not make the noise when revving the engine in neutral or park. The noise also stops if I shift into neutral while rolling down the road.

The noise only occurs when I am in gear and accelerating or holding the gas steady. It is intermittent and goes away after about 20 minutes of driving. It seems to happen more when I am driving at a low rpm in a high gear, or it often starts when the transmission does one of these "half-shifts." Once the squeak noise starts, it will continue (getting louder and faster) until the transmission shifts again or I let off the gas.

Any thoughts on what this sound could be? I tried to use a video camera to capture this noise, but the camera could not pick up the sound.

Some info that could be relevant:
1994 E350 with 5.8
Transmission was rebuilt about 10k miles ago
I had the transmission out about 3k miles ago when I rebuilt the engine. I installed a TransGo shift kit, replaced the fluid (Dex/Merc) and filter at that time. The noise seems to have started when I put everything together 3k miles ago.

Thanks!
Dave