Ok so I'm having a problem that seems to have been getting worse. 1999 7.3, auto, 4 door, long bed, 6" lift. When I got the truck a while back I had a slight problem where if I was accelerating hard I would hear a clunk when it shifted into 3rd gear. I'm not hard on the truck that often so it took me a while to realize that the noise was the driveshaft hitting the floorboard right at the joint at the transfer case output.
The carrier bearing was flopping around a bit so I replaced that first. No better... kept driving the truck since it was only a problem while being really hard on it. But it's gotten worse so now when I tow (usually not more than 7-8k lbs) I have a bad vibration on takeoff at low speed. If I try to take off from a stop very hard with any load the driveshaft jumps into the floorboard. So I just replaced the transmission mount and checked the motor mounts, no better. Any ideas or experience with this problem? Only thing I can figure at this point is axle wrap...?

 

Is the carrier bearing shimmed down to correct the angle from the lift that will cause vibration on take off. Are you sure the drive shaft is hitting the floorboard and the slip yoke doesn't just need greasing have you ever greased it before I don't see how it's possible it could have that much play in it and still be driveable at all especially with all the stuff you have replaced. You have likely trashed your new carrier bearing from all the vibration and improper angle if I were you I would make sure the carrier bearing is shimmed properly and grease the slip yoke and see what you got. Do you have flat or tapered rear blocks.

 

Have you tried greasing your slip joint?

 

The carrier is shimmed, looks like about 1 inch. It is still solid actually. Yes it is absolutely hitting the floor, there's a nice dented gouge spot where it hits at the first joint at the t-case output. I greased it when I had the driveshafts out for putting the carrier in.

As for the blocks, they are stock 4" block as far as I can tell. There is a really small block sitting on top of that less than an inch. Does not look tapered. That said, the pinion looks straight with the driveshaft.

I am going to have someone power brake it for me later today so that I can watch what is going on under there. Not sure what else to do

 

I'm having a very hard time envisioning how a driveshaft that is still connected at both ends could flex up and hit the floorboard.

 

Have you checked the outer cross member bolts that hold the transmission mount and carrier bearing cross members to the frame? How about the rear axle to spring plates and U-bolts? Something has to be loose or broken unless you have a rubber driveshaft...

 

All of the crossmember bolts are tight. Is the crossmember adjustable up/down? I'll have to check when I get back to the truck.

I will check the spring plate bolts next, thanks

 

I will see if I can upload a picture for you of where it hits

 

It almost sound like axle wrap, but I've never heard of it near the T case.

 

Broken tranny/transfer case mount?

 

Yah don't check the bolts, check the mounts themselves.

 

This has to be a broken transmission mount. Probably a broken or close to broken motor mount as well.

 

Here's a picture of where it's hitting for y'all... see that shiny metal spot above?

 

Literally just put in a brand new transmission mount 2 days ago. I thought that would clear up the problem, but it hasn't. Motor mounts are not broken, but the drivers side one does have some flex when power braking.

 

So I got to watch everything under the truck while power braking... The most movement out of anything under the truck was the rear end. The weird part to me is that it looked as if the drivers side of the axle wanted to come forward more than the passenger side did. Not sure if that was the case or just my eyes. Either way I am going to check all of the bolts in the rear and see if I find anything. The drivers side motor mount has some flex in it but is not broken or even close to it. I'm thinking I will replace that anyway since it will need to be done at some point.