Thanks for the input.  That was one of the first things checked.  Hitch
is a Pullrite SuperGlide 16.  All bracketing is customized by Pullrite
for each specific vehicle.  When I ordered the hitch, they wanted more
info on the truck than I ever dreamed they would need.  Fits great!!
The hitch installer from the trailer dealer test drove the truck and I
sat in the back seat and watched the hitch.  NO front to back motion
there.  The problem seems to be the fact that the overload spring is
only being hit on the rear end of the arch.  That tends to rotate the
entire axle assembly backwards.  Imagine the truck sitting still and the
axle being rotated backwards.  That would move the truck backwards, even
though it may only be an inch or so.  Now imagine the truck being moved
back and forth an inch or so while doing 60mph.  Quite an experience!

We are attmpting a fix by mouning a rubber bumper stop to the front end
of the spring so that both ends of the spring come into contact with the
frame stops at the same time.  Tried it by using a large hose clamp and
a 2X4 on the end of the spring.  Big help but not quite there.  Just
talked to the hitch installer and he said the biggest rubber bump stop
he could find was about an inch too short.  He's going to install a
spacer between the spring leaf and the rubber bump.  I'll let you know
how it turns out.

Still open to other suggestions in case this doesn't work.

Bill

Thanks for the response, Mellowed.  Somehow, my original post didn't
show up, so I reposted and a few minutes later, my first one appeared.
Like you, don't know why but I have two posts and getting responses on
both :-)  Air bags were the first thought of the RV dealer but since I
have a short bed, had to go with the slider type hitch.  I chose the
Pullrite SuperGlide which is an automatic slider.  The tighter I turn,
the further back it slides.  Great hitch. Bad part is that the brackets
for the hitch go right where the air bag would be, so that didn't work
out.  The fix for the spring hitting the back end first and causing the
jerking was solved by adding to the front bump stop.  Now both ends of
the spring touch at the same time.

Bill

Yes, you are over gvw for the truck.  Don't know if that's what's causing
the jerking issue, but it could be.
My 250 with trailer on grosses out at 9200, which is over the 8800, but am
using a reese slider, and it's 6" in front of the axle in the tow position,
and have no jerking at all.  If I try to tow with the hitch slid back, (4"
back of the axle) the thing is terrible -- it goes down the road like a
porpoise, the slightest bump makes the truck and trailer bounce just as
you've described.   I'd still look at the position of that hitch, if the pin
isn't in front of the axle by a few inches, you'll have these problems.

The other thing is that you didn't post the truck weight without the trailer
aboard, so don't know about the pin weight.  It might be too heavy.

Good luck with it, and happy camping

Bill