Hey All,
So I am wondering if anyone has experienced anything like this. My girlfriend has a 1991 240 Wagon. I don't know most of the history of the vehicle unfortunately as she bought it with at least 200k miles (the odometer stopped working an unknown amount of miles ago).

A few potentially-related known problems.

1. The overdrive is out, it has kicked in maybe 3 times over the past year and a half so she has just tried to avoid driving on the highway as much as possible.
2. When she bought the car the rear differential was leaking and noisy. So a year ago she took the car in and the mechanic apparently replaced the leaky rear differential with another used (I assume) diff. Apparently this fixed the leak and the noise for a while.

The other day I took it out to run a couple errands. Regrettably, I took it on the highway. I was trying to see if I could get the Overdrive to kick in and in the process, reached probably about 60, maybe 70mph. I began noticing a whirring sound that seemed to be coming from the differential. It very quickly and progressively got worse within a few miles of highway driving. I think I also noticed a burning smell - this is hard to know though as they were repaving the highway. I pulled over and it looked like the diff was leaking. The liquid was burning off from the hot metal of the differential.

In its' current status, the 240 has a slight whirring sound when accelerating. It seems to be coming from the main driveshaft > u-joint > rear differential area. However, the most alarming sound happens when you let off the gas and coast at speeds above 30 or-so mph. There are a couple small clunks and bangs as you let off the gas, followed by a loud whirring and squeal, coming from the rear diff area. It also vibrates and feels like the car is going to come apart.

I was able to jack up the rear end and record a video under the car. With the rear wheels jacked up, we put the car in drive and gave it some gas to see what is going on under there during accelerating and decelerating. I recorded video and have posted it below. The quality makes it difficult to distinguish the different sounds happening, so I made some equally crappy graphics that probably don't help much. I'm hoping a seasoned mechanic can maybe see something in the video and help give me a diagnosis?

Any help is much appreciated! Thanks!


<iframe width="560" height="315" src="https://www.youtube.com/embed/3iIUaUXaPiI" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

 

jack up one back wheel, put it in park, get someone to turn the wheel to its limit either direction, while they keep doing that, feel the Ujoints for backlash if they feel loose change them
mark the driveshaft and keep relationship as is, or you may put it out of balance.
you may have a bearing near the middle supporting the driveshaft that might be bad,
if you need to replace them id pull the driveshaft out
i usually press them apart using various sizes of sockets for "tooling '' and a bench vice.for my pushing.
good to wash it well first.
good time to check the trans. rubber mount too.

expect some lash in rear axle. i suspect a bad ujoint. if it's ok you won't feel any play in the ujoints. if they wear sometimes they loose rollers, then driveshaft goes way off balance since its not aligned. that will make clunking noises and shake the car.

if the diff has limited slip maybe you need both back wheels up but you get the idea,, you might stick a scrwdriver through a ujoint and detect play, if alone.
don't loose the relationship of the parts, I'd paint a line on one side first before any disassembly its balanced as a unit. unbalancing it will cause vibrations.