1999 FLD 120 Rear end keeps going

This is the problem, 100%.

I wonder what he’s running for power?

 

Well not exactly but a tandem rated at 34,000 has 2 rear ends rated for at least 17,000 lbs each.
Old Blue over there to the left is a single with a 250 Cummins. I thank it has a 22,000 LB rear. It is physically larger than standard tandem diffs. Look at those Consolidated Freight wiggle wagons with a big Cat. They have a big diff under them. Do not have to tote the load just have to be able to pull it.
It has been a long time but folks would take a single and add a little frame. Then install an air bag tag. Done correctly they did well but those old trucks did not have a lot of power.

 

True but OP's truck wasn't originally built as a single either. If we get some details as to what the failures have been we'll get a clearer picture.

 

No

 

I’m assuming that you’re single has a axle lockup if so if it’s plumbed wrong that would cause all kinds of stress on rear end and lots of heat .. do you wear tires quickly or hear tire chirping during turns?

 

My first question is… with the suspension working up and down, does the slip yoke have enough travel in the splines, or is it bottoming out as the suspension works putting impacting pressure on the input shaft bearings….?

 

Just my .02, I knew a driver that kept having rear end problems, he went thru like 3(under warranty) and they finally took a closer look of what could be the problem. It ended up being a bent axle. So they replaced both and the problem was solved.

 

Fill it full of milk of magnesia.

 

Bent axle was my thinking, too. Another's possibility: driveline angle?

 

Driveline angles that are off takes out the U-joints. Been there. It never took out the rear end.