I've had a thump on acceleration from a dead stop in the rear of my 96 ever since I bought my truck a year ago. Every brake part on the rear of my 96 is new inlcuding the flex hose, cables, tried 2 sets of shoes, drums, hardware, cylinders, and steel lines yet I still had that annoying thump under acceleration from a dead stop Today I replaced the rear flex line - it was still there....I decided to pull my rear axles again and compare them to those in my old 88 parts rig....

The 88 was definitely in better shape concerning the spiders, side gears and the axle splines....The 96's were worn pretty good. So I swapped everything between the 2 trucks....Sounds much quieter and the thump is less noticiable but still there. The U-bolts are tight (new rear springs 1.5 years ago), the u-joints were new a year ago (spicers), all that's left that I can think of is to replace the lower rear sway bar bushings...they are getting bad....Frustrating as hell this is. I've been chasing this thump for a year now...Any other idea's???

 

I had the same thing on my 94, for me it ended up being that the output shaft of the tranny and input shaft of the T-case got worn so there was play in between them. So I had to have both the Tcase and tranny rebuilt. Also when I had my truck regeared the shop that did the work said that the shaft that connects the spider gears to the carrier can wear and that mine wasn't very bad for the miles on the truck. But that shaft is what they first thought was the problem when I ended up having the tcase and tranny done.

How about all your bearings (carrier, pinion) maybe one of them is going out?

 

My 85, my brothers 86, and my dads 92 all make a clunking noise when accelerating from a stop. Guess it just something these trucks do?

 

My bearings all feel tight, no slop....The clunk is further back than the tranny, feels like it's are the rear wheels. I've heard about the tranny output shaft/yoke deal before, but I pulled mine out and it looked ok. There is some wear but it moves freely...I still think it's in the rear end or something out back...It's driving me #$%^&**&^% nuts!....LOL

 

Make sure the spare tire is tight. Look for loose cargo under the seat or in the bed. Then try to duplicate the noise with the truck on jack stands. A trustworthy assistant operating the pedals will make that a lot easier. Use a mechanic's stethoscope to listen inside the axle. Check the rear wheel bearings, carrier bearings, & pinion bearings. These trucks are known to back off the pinion nut.

If you can make the noise on the road, but not on jack stands, turn it off with all 4 tires on the ground & see if you can make the noise by pushing/pulling the truck back & forth.

 

have you checked the transmission mount? just curious.

 

I pulled the driveshaft, greased the splines and bingo - the thump is gone....I got this tip from another forum so I tried it.

 

My '06 Lariat would feel like I was going through a large pothole on takeoff sometimes. It felt like the rearend was the culprit but the dealer ended up doing about 1500 dollars worth of transmission repairs to fix the thunk. At least I got the 30,000 mile trans service done for free. Now the truck has over 37,000 miles and it has never done it again.

 

I greased the output splines on the t-case, problem solved...I might have to do it every so often but that's no big deal.

 

I think you might. If the grease got out of there from the factory and caused the thumping, chances are it will happen again.

There was an article about this in Popular Mechanics mag a while ago, said it was a fairly common issue for trucks in general when driving with nothing or little weight in the back.

 

Yup, and it drives you nuts!!!....I chased that thump for a year and it's finally gone.

 

They didn't use the right kind of grease at the factory. There's a TSB (or maybe FSA) about using Teflon grease for this problem. Browse around the MotorCraft Lubricants site & you'll find the one that's recommended for this application.