I have a 2003 Excursion with a 6 inch lift. I have what feels like a tail wag from the rear (no trailer towing). I am running a Hellwig rear sway bar, and have everything really tight. Just had the front drag link and tie rod replaced. My ball joints are all tight. I just had custom arched springs installed to get rid of the blocks in the rear, and am running a 4 degree shim to minimize harmonic vibrations. It has always had loose steering, so not sure if the new rear leaf springs caused this to feel more pronounced with having the rear end lifted higher than the front. If I had to guess, the rear end sits about 3 inches higher than the front.

One of the main culprits I am still looking at after tightening up the stock steering gear box and still having play in the steering wheel, is the steering gear box itself. Could a bad gear box cause over correction and make the rear end feel like it is wagging?

Oh, and I am running brand new Cooper M/S 10 ply 265/75/R16 tires with 55 PSI front and rear.

Any ideas?

 

If those new tall springs out back aren't of a high enough spring rate they will invite a tail wag, that was the issue with the factory springs, too soft. Not all but some EX suffered from serious tail wagging due to these soft springs, mine was noticeable running solo but was pretty scary while towing. Stronger springs and a Hellwig tamed mine.
Must look awesome with that much lift and stock sized tires, may also contribute a little bit to the wag, tall and skinny......

 

Another simple thing to check is the play you have where the spring sits in the rear hangers. The bushings on my springs were shot and I had a lot of rear kick (but more on the towing side). You can grab a pry bar and put it in the hanger and see how easy it is to move the spring from left to right.

 

I would definitely try some more air in those tires for one. 55 in my rear tires with the lift doesn't feel good. I like at least 65 in the back running with no trailer, and 80 if I'm towing.

 

With that much rear rake, the front tires might have more caster and camber thus be more sensitive to edges/seems on the road. Front tire wear and air pressure also make differences as well..

 

Thanks Christina, I will try 65 and see how she does. The other issue I now have is harmonic bounce when on the highway driving over the expansion joints in the road. My wife hates it, and I don't blame her especially since she is pregnant. Any recommendations for keeping the rear end from bouncing? I am running new procomp shocks in the back, but thought maybe adding dual shocks in the rear might help. I thought about a traction bar, but I really don't see how that would help at all. Oh, and I do have a Hellwig rear sway bar.