Vibration turning in reverse after accident

Back in November of last year, I was involved in a hit-n-run accident. Long story short, guy tried to make a right turn doing about 40 mph in a Kia Optima. He couldn't do it without bouncing off of my driver side doors. He did hit the DS rear tire and rim, but just enough to put a rub mark on each. No damage otherwise. The guy tried to run, but I chased his ass down. The body has since been fixed.

Right after the accident I noticed a strong vibration when turning in reverse. It only occurs when the vehicle has been sitting for hours. If I drive it a few miles and try it again, all is smooth. It DOESN'T start real slow then progress faster with speed. So that eliminates the tires. It's a pretty consistent vibration though it does speed up a little with speed. But I'm talking between 2-10 mph.

The guys at the body shop says it feels like the rear diff is stuck in the lock position. A buddy of mine who's a master tech told me to start with checking the parking brake to make sure it's not over-tightened. He also said there's a small chance the axle is bent. But that can only be determined with a dial indicator against the axle.

Any other ideas?

 

Update: The axle was given the a-ok by a local Toyota dealer. Checked the brakes and everything was fine there too. I started noticing the vibrations after the accident so I thought it might have been related. Dug a little more and remembered that the last time I replaced the oil in the rear, the auto parts store didn't have the dino valvoline gear oil I normally use so they gave me the synthetic one at no extra instead. It ran fine for months so it never crossed my mind it might be the culprit. On a hunch, I decided to swap out the synthetic with regular dino oil. Sure as shit, that was the problem. Runs smooth again. Lesson learned, avoid synthetic oils in the rear diff if you have an LSD.

 

Its not so much the "being synthetic", its that the synthetic you got probably didn't have enough LSD additive in it, assuming it had any at all.

 

Not enough additive was not the problem. It had the LSD included plus I also threw in a 7 oz. TransX Posi-Trac additive. There was enough, believe me. Had the same thing happen a few years ago when I tried the Mobil1 synthetic. That time I added a second tube of additive and it still vibrated. Didn't stop until I swapped it with dino oil just like this time. That's what gave me the idea to try dino again. Regular dino oil with no added LSD additive and a 4 oz. Lube Gard LS Supplement and it's working great.

 

I've never liked mobile synthetic. I wouldn't wish it upon my worst enemy. Very bad stuff for transmissions and basically causes synchronizers to not work AT ALL. Probably the same reason it won't work in a manual transmission is why it won't work in a differential. I'm sure that synthetic would be fine, as long as you use a GOOD synthetic. I've been running amsoil in my transmission ever since the very brief attempt at mobile.

 

I use Amsoil in my tranny as well. Works great. So far I've tried two synthetics in the differential and both failed. Mobil1 immediately and Valvoline over the course of a few months. I have no issues using synthetic oil in other places. It's just the LSD diff that seems to hate it.

 

Its probably one of the reasons that nobody uses mechanical LSD's any more (cost being the other). The simplicity and reliability of adding that function to the ABS is unbeatable.