Hi Folks,
I fixed mine in an unconventional way:
https://www.josephrampolla.com/44tonfix.html
In my case the lower frame pieces had become loose. That caused the armature to bind. I cleaned the mating surfaces of the black plastic piece (underside of truck), applied a super glue for oily surfaces, then made clips to also provide a firm mechanical hold. Make sure you have the ball-bearing at the bottom as in Rob's post above.
Photos before things were cleaned up:
Hope this helps!
Take care, Joe.
P.S. The repair has held since November of 2011.
Here it is in a recent video: