Well if that pictures shows what I think is the front driveshaft to trans output flange, then it's not a driveshaft problem. It has to be a trans mount or trans mount cross member movement issue. Or your cab is moving around too much on the body mounts.

 

I was going to suggest cab mounts from the beginning. The floor should not be that close to the driveline. Does your bed and cab line up correctly?

 

It lined up straight with the bed before I put an aluminum flatbed on it 5 months ago. I'll going to try to look at another truck today and see if I can find a difference in the driveshaft-to-cab measurement

 

With this truck being nearly 20 years old, it's possible that someone has done a conversation of some sort, changed from manual to auto or swap transfer cases, maybe used the wrong crossmember. Just throwing ideas out there.

 

I hear ya... it's automatic from factory. I got it 13 months ago right at about 220 miles and it just turned 296k 2 days ago. The problem was there when I got it except the only time it hit was when it slammed 3rd gear at high throttle after I put the TS 6-position on it on the high tune. I don't run it on the high tune, so I just kind of ignored it until it's gotten worse now and is accompanied by the vibration on hard takeoff.

 

Just thought about this, I assume when you are checking the mounts, you are putting a prybar between the crossmember and the mount. Could the other side of the mount be the problem? i.e. The actual transfer case or transmission casting boss be cracked or broken and you aren't seeing it because of the way you are checking the mounts?

 

Checked with a 30" pry bar... and watched it while power braking to see if anything had flex. The trans mount that I just replaced was so far gone that I could flex it just by pushing the t-case with my hands. I've been busy with working and just using the other trucks, but when I get a chance I'm going pull the rear driveshaft again, recheck what the slip joint looks like, check the rear spring bolts, check for any problems inside the diff also that could cause any kind of "jumping" action...

 

Another vote for cab mounts that looks really close to the cab.

 

I'm with WE3ZS. Although I can see the dent on the underside of your floor, I cant see how it can be done by the driveshaft as you're driving or underload. It doesn't make pivotal sense. Something must be flexing between the rearend, carrier bearing or mounts to allow the driveshaft to even hit that spot. Are you going over rough terrain and tweeking (4x4'ing) the truck when you hear the clunk? We're assuming a lot of things here. The carrier bearing shim and drop amount is very important for alignment. You sure the right trans mount is being used?

 

Can someone that knows these transfer cases say if it has a bushing in the rear output shaft housing? If it does and it's severely worn, could account for the vibration as well. They bad carrier bearing could have caused it.

 

I would get under there asap with a wrench and check the transfer case mounting bolts.

 

 

 

Its always thoughtful when the OP comes back to let us know what it took to fix the problem. Free reps to you!

 

I actually read this thread yesterday and hoped you would come back and let this guy know what the fix was. Good job OP, reps coming your way.