I have a 99 F150 Ext cab 4wd with the 5.4 and auto tranny. In the last couple days I have been doing a fair amount of driving with it, I drove it about 250 miles yesterday and half way through the trip I thought I heard a noise coming from under the cab after I hit a bump but it went away in a matter of a half a mile and never did it again so I didn't give it much thought because I was on kind of a rough road. Today I was on my way to my moms side for Thanksgiving about 15 miles away from my destination I hit a bump and the truck started making a growling sound the sounded like it was coming from between the seats under the cab. I pulled over and didn't find anything wrong so I got back in and drove a little further down the road and it kept making the noise so I pulled off on a gravel road away from the highway where I felt safer about crawling under the truck and tried giving the drive shaft a turn to see if I lost a u joint which I discovered not to be the case so I got back in the truck and limped it the rest of the way to my aunts house, Where I did a little better inspection. I found that I can grab the drive shaft right behind the u joint at the back of the transmission and move it up and down approx an 1/8 inch. The movement was not in the u joint but in the collar that goes into the back of the transmission and discovered that my back seal appears to be leaking that goes around the slip yoke where it goes into the transmission. So I left the truck there and rode back with my brother. I'm taking another truck and trailer down tomorrow to get the truck and trailer it home. I deemed it not fit to drive for fear of severely damaging anything if its not already to late for that. This truck has always had a clunk in the drive line that I was never able to locate. It just did it at completely random times mostly when I would stop completely then take off, or just stop kinda quickly then then ease off the brakes. This is actually my fiancee's truck. A week ago she was going to take it somewhere that was a pretty good drive so I had checked it all over then because it had an issue the week before with the elcheapo bonded brake shoes falling off and her assaulting the rear end of my diesel with her truck because it failed to stop. So I did not want her to have any problems when she was out and about by herself and at that time everything was great on the truck no leaks no anything and now a week later this... So my question for you folks is this What did I break and what went wrong? This is the 2nd one of these trucks I have owned and been around a couple others that family members own. I absolutely love these trucks but they have gave me many head aches but this is a new one for me. Thanks in advance for any advice or input folks and I apologize for writing a book every time I post, Just trying to be descriptive.

 

I believe its called a tail shaft i have a small amount of play in mine as well. Maybe not an 1/8 but a lil. And while i had my truck at the shop getting the tranny flushed i asked them to check it out. He told me as long as the seal isn't leaking its good to go but if it does leak its gone bad so your going to have to replace it.

 

my dads supercrew has a clunk upon acceleration and any other time you surge the drive line.. wich sounds similar to this.. still havent had time to crawl under there to inspect..