Just to follow up on this problem... when I got the old final drive out of the unit, I noticed a couple of things.
The input to the drive is a female spline hub and the splines were worn down to just a sliver left, she wasn't going much further. The male spline on the traction motor which drives this was worn a little but must be much harder material.
This is at the 14,000 hour mark approx.
This play between the splines can actually be readily noticed by a "clunk" when plugging.
So if you are hearing a clunk at this hour mark you should be getting a gearbox ordered up.
Also - the input bearing (not the radial bearing) that is supporting the female spline hub/gear, was almost seized and was locking up at times.
When this bearing locks up, the traction motor rotational inertia/torque can actually overcome the holding action of the steer motor assembly and "whip" or throw a very drastic steer action - very suddenly causing the truck to veer wildly and unexpectedly!!
Just throwing this tuff out there - maybe will help others.
Thanks to all for your inputs!!