As usual I started with a search here before posting and I only found one thread that sorta touches on what I'm experiencing. '97 w/ 9.75 LS rear end.

First, this rear end clunk has only occurred twice. Last night and today. Both times, the clunk occurred in similar scenarios. I start breaking early for a stop light. As I'm slowly coasting toward the light, I'll apply the breaks to bring the vehicle to a stop. It seems that as the vehicle is stopping, I'll experience a clunk/push feeling that sends the truck forward - but the breaks hold the vehicle in place. In other words, its almost as if the vehicle has been rear-ended because I can feel and hear this vibration throughout the entire truck. However, it does not occur every time I break.

I'm concerned because I know there is a definite problem at hand. I'm guessing something with the limited slip clutches or U-joints.

Fire away...

 

If you are stopping in a straight line it doesn't seem like it would be limited slip clutches as they should not be doing anything. I have had clunking sound happen when there is a delayed shift back to 1st gear. You might want to consider your tranny.

 

Good thought. I just went out and checked the tranny fluid with engine running and in park. Level is in hot range (just finished driving) and color is good.

Another thought: maybe brakes grabbing even though I've felt it twice in a rear-end.

 

Well if it was a sticky valve in the valve body, good looking fluid may not affect it. The feeling of being pushed seems to indicate power being applied to the drive train. Sticky brakes would more seem like you hit something with the front. U-Joints would probably clunk when going from power to coast.

 

Rusted through trailing arms ??

 

I would check the trailing arms first, also check to make sure that the rear calipers are releasing properly. While you are back there check or change the diff fluid be sure to use the friction additive from ford ( the worst smelling substance on earth).

 

I checked the trailing arms. All looked fine. No rust through. I went for a drive again and could not duplicate the clunk. I'm honestly at a loss. I have no idea. Changing the diff fluid is a good idea. Hopefully, I'll be able to tackle that this weekend.

 

The sudden burst of power would backup the shifting into a lower gear theory being thrown at you. Is your idle running high, say 1,100 or more?

 

Nope, idle is fine. Normal 600 rpm range at idle. This all started last night and I've been hard pressed to duplicate it. I went out again and attempted to duplicate the clunk without success.

Any other ideas are appreciated.

 

Non repeatable problems are the toughest to troubleshoot. If you get a good idea how do you try it out and see if it worked? I hope you can figure it out. I tend to wait and see if it get bad enough to really bug me, or starts to happen all the time before I worry about it. Your plan of "attack it early" is probably a better one, just more difficult.

 

I've decided to change the diff fluid front and back just as a precautionary move.

When I look inside the rear diff at the gears, is there anything I should be looking for besides metal shavings, etc? Pictures or diagrams would be appreciated.

 

I`m having the exact problem to the letter with my 06 Navigator. Has anything you tried fixed it yet? Just looking for a starting point. Thanks