To check drive line slop Set E-Brake on level ground put truck in 'N" slide under truck and twist drive shaft right and left. If you have more than 1/8 of a turn it's boarder line excessive and should get your Differential checked. It's not a safety issue it just amplifies your clutch chatter...

 

It has got to be the clutch, I might even say possibly a broken torque spring in the disc or a broken finger on the pressure plate!!!

 

I have the same problem hauling my 48ft gooseneck. New universals. stock clutch 45k on truck. Bounty Hunter if you find a solution please post!

 

My mega is doing this bad too. Has always done it with the 40' jayco toy hauler. I tried the one piece drive shaft, and it made little to no difference. I have the Hellwig over loads, plus the air bags. Hellwigs are as tight as they go, so that should at least help with the axle wrap. I think its the clutch honestly. My truck had the big double disc Valair in it when I got it. I just ordered the new south bend today to see if thats the culprit. If its not, I'm going to be crazy upset.

If I put the truck in 4 low, to back in or inch ahead, the problem is not there at all. perfectly smooth and jerk free