Good evening, this is my first post and I’ve searched and wasn’t sure if what I’m reading is what is going on with my car.
I have a 2006 V6 automatic with 155,000 miles, it’s got a Yukon LSD and 4.10s. I just put C&L lower control arms and Lakewood 50/50 drag shocks on it and got a differential flush since there’s a leak and it was dirty fluid and low.
Since I got the differential flushed, I’ve noticed a thumping feeling when I hit the gas going from a coast over about 20 mph. I’ve checked the U-Joints and they seem fine, the pinion on the transmission end shakes a little but I’m not sure if it’s regular play or not; there’s no fluid leaking. When I get a chance I’m going to go through the suspension I put in to make sure everything is tight in case that’s it.
When I got the differential flush they put in synthetic 75W-140 with an additive, and this particular LSD advises against using synthetic oil.
Could what I’m feeling be the LSD acting “aggressively” as the manufacturer cautioned? I’ve had this differential for 4 years and haven’t had a problem, even when leaking with dirty fluid.
I’ve also noticed that occasionally there’s noisy clunks shifting but I’ve tossed that up to my car kinda being a POS. It’s also at the mileage of a recommended transmission fluid change, so I’m thinking maybe that could be a cause.
My apologies for this essay here. I’ve had a lot of thoughts since I’ve noticed this thump feeling and I was hoping someone knows what I’m talking about and can shed some light on what it could be, and if anything I’ve done could have an impact.

 

If the thumping occurred after the fluid change, then that's likely your problem.

 

It seems like the most logical culprit. I was overthinking since it didn’t make any noise turning when the LSD would be working harder, but at higher speeds it also seems noisier which indicates an oil problem