Investigating possible driveline issues.

Need some help. I didn’t grow up working on vehicles but I’ve done everything I could to my truck as far and service and repairs as long as I have owned it thanks to this forum and YouTube. Over the years owning it, I’ve replaced this or that in order to address one issue that has been bothering me for the 3 or so years since owning it and I still can’t solve it. I’ll do my best to describe it.

2007 4.0, 4x4 160,000 miles.

To put it simple, the front end/wheels seem to rock forward or back when I start and stop respectively. For example when I approach a stop light and when the wheels stop rotating I’ll feel a slight jump/bump forward in the front end just below my feet. And when I accelerate I feel that same drop/bump back as the wheels begin to rotate. Occasionally when traveling at speeds greater than 40 mph I’ll coast then re-engage the accelerator and I’ll feel a catching/jerk sensation.

When I first bought it I replaced a crappy unknown lift suspension with an OME system front and back and new SPC UCAs. Problem remained. About 2 years ago I had some u-joints go out. Replaced those. Problem remained. Changed rear diff oil last year and found some shavings considerable in size. Wasn’t able to address it as I was transitioning out of the military. Figured the rear-diff issues would present themselves more clearly but I haven’t noticed anything (I think).

I’ve jacked up the front end multiple times to check movement in the bushings or bearings and everything feels 100% solid.

Here’s the odd thing, when I put it 4x4 high or low the sensation goes away. It’s as if the truck becomes more solid. I’m now starting to think it could be the rear diff but none of the symptoms sound typical that I’ve seen on here, no noise or grinding. Could the sensation in front end be part of the drive shaft catching/releasing on start and stop with a gap due to the wear in the rear diff?

happy to hear any suggestions or clarify my attempt to describe what’s going on. Love this community so thanks in advance.

 

Sounds like a combination of driveline slack and axle wrap.
My truck is the same year as yours, I did have the TSB springs installed about 6 years ago.
They are starting to get “soft” and you can notice the axle wrap more now.