Your situation Sounds worse than mine

 

I have a 08 Limited AWD with LSD. I am experiencing the same issue, I think the only time it doesnt do it is when its a slow take off or all the slack is out of the drive line. I have inspected the drive shaft and rear pinion for slack but have found none. I may talk to a tech at ford or just bring it in since I am still in the 5/60 powertrain.

 

dammit, i have the same thing. i definatly dont have 800 dollars.

 

I recently had that problem, It went away by it self.
Saturday I noticed a line of gear oil leading out of the driveway, then I noticed a hole in the diff cover, pulled the diff cover and a bolt had backed out of the ring gear.
fixed hole refilled lube and now its better then it was before the drive line bump.
If you have the ability pull your covers retorque everything and fill it back up.
I was lucky I didn't blow the thing up.

 

Mines doesnt leak at all

 

The thump you are experiencing is lack of lubrication in the slip yoke. The dealer is probably asking $800 because that is the price of the entire drive shaft. You can repair this issue for free by doing the following:

1. The slip yoke is inside the rubber boot after the carrier bearing and before differential.
2. Remove the boot clamp closest to the carrier bearing. If you're careful you may be able to reuse clamp, otherwise a new one is about $12 or you can use a zip tie to seal it back up.
3. Remove four 12mm bolts that attach the drive shaft to the differential. Before you remove the shaft, mark one corner of it and the round input shaft on the diffirential so that you may install in the same position.
4. Grab the shaft and push it towards the carrier bearing and you me need a large screw driver to pry it off diffirential.
5. Once it breaks free from diffirential, the opposit side will slide right out of the slip yoke flange.
6. Grease the splines on the shaft and on the flange that is attached to the carrier bearing. I've used thick bearing grease and its worked well. Ford uses teflon grease but its difficult to come by and doesnt seem to work all that well anyway or we wouldnt be reading/writing this post.
7. Insert shaft back into slip yoke flange. You will need to roll the truck a little back and forth to lign up the bolts (of course do this on a totally flat surface).
8. Replace bolts and clamp. This will immediately resolve the thump.

You may need to repeat this annually because our drive shafts simply suck. I'm going to attempt to install a grease fitting on my slip yoke flange this weekend so I can lubricate as needed without dropping the shaft. If weather permits and I'm successful will post pictures and instructions.

 

Primalurges...How did it go? Think I'm gonna try this. Started acting up just this morning.

 

Well I greased the slip yoke and haven't had a thump in a month. I changed my mind about adding a zerk fitting because someone I know that drills metal frequently told me that the yoke input shaft is very hard a may crack when drilled. I'm not 100% convinced of that theory but with a repalcement drive shaft at $800 and my lack of a drill press, I couldn't afford to be wrong. The last time I greased the drive shaft the bump went away for over a year. I guess I can deal with taking it down and greasing when necessary.

 

I had a pretty gnarly thump in mine too. After I finished installing my leveling kit, I dropped my driveshaft and greased the slip yoke. Probably 12 cents worth of grease and half an hour later, no more thump. My truck has 96,000 miles and there was no grease on the splines.

 

I had the exact same issue as described by the thread starter. Was quite worried it would be an expensive fix.

Did the procedure as described by primalurges this morning and no more thump thump!

Thank you so much!