2001 WB Transmission Whining

I have a 2001 WB Chevy Venture with about 110,000 miles. While I was visiting family in New Hampshire last week the van began to make a whining sound after 40mph. After some experimenting it looks to be associated with the 4th gear. When I put the transmission in neutral and step on the gas the whining gets louder until the mph gets below 40 and then stops. The whining is also not present in 3rd gear. The trans fluid looks red and doesn't smell burnt. I had to drive the van back home to Kentucky which was over 900 miles and all I had to deal with is the annoying whining. No unsual jerks and smooth shifts. I did notice an increase in fuel consumption, so I can assume that it isn't shifting into 4th gear. I wish I had a tach then that would be easy to figure out, but I haven't yet gone and listened to the change of RPM. I have read some posts that this could be associated to a sensor or something stuck in the valve body. I can do minor work, like changing the trans fluid and look at the valve body. I am more of an engine guy than a trans guy. Does anyone have any suggestions on how this can be fixed? I am hoping to avoid an overhaul since my wife totaled my vehicle last month and this is all I have to drive.

Problem found. It was my overdrive shaft. The splines on the end had stripped off so I wasn't engaging the overdrive clutches. Apparently this is pretty much commonplace since on earlier transmissions the splines weren't heat treated all that well. The downside is that the transmission was pretty worn out and also with the metal shavings associated with the shaft splines sheared, so I needed it replaced with a remanufactured one. That is in process as I speak. Hope this helps someone

The old "cookie cutter" strikes again:) The term in tranny biz for the 4th clutch shaft.