Axle windup on 1999 f250 with 6 speed manual

I have a 1999 F250 super duty with a 6 speed manual and a 7.3 diesel.

When reversing or taking off forward, I get a lot of jerking especially when I have my 10,000 pound fifth wheel attached to it. The clutch was replaced and the flywheel resurfaced and the problem became less but within a few weeks it was just as bad. I had an extra leaf put in each side at the rear and the shocks are new as well.

Any suggestions on how to correct this or is it the nature of the beast?

Thanks,

George