Well sometimes you learn the hard way, if so I try to pass it on to someone else so they don't go through the same thing.

First quick history on my 1988 Bronco w/355L rear and 4 speed granny gear tranny.

1) Was always real noisy (figured that's how Bronco's were).
2) Spent a lot of time and a few hundered to quiet it down. Bought half inch thick carpet underlay. God knows how many square feet of Dynamat (actually a competitor of theirs Tsunami) and laid it inside the rocker pannels, rear bed. Even several cans of Bondo rubber underlayment. Did it get quieter? Yes, it certainly did, but now:

3) A loud low frequency noise. Sounded like exaust figured that's what it was. I went over every single joint, even isolated the muffler from the metal bracket with Hi temp silicone hose. Made a little improvement, but the noise was still there. Especially under acceleration in a higher gear.

4) Then it got wierd. When driving home on the highway and went from flat to downhill at 55+ mph (translation = reached a point of no load), the rear of the car vibrated like mad.

5) Didn't take long to realize that if I put it in neutral at 50+ mph it vibrated horribly. I was ready to try to rebalance the driveshaft myself when I brought it to my trusted mechanic. They put it up on a lift, in gear and watched from beneath.

6) Diagnosis: Double cardian joint worn out, needs repair. Their top mechanic rejected the idea of doing it himself - he had a bad experience. I was told to leave the car for a day they would send to a machine shop.

7) Received car back with rebuilt double cardan joint (they sell a repair kit) and a new rear U joint.

8) Wow - what a difference much smoother, even the rattles from the transmission were gone. But at 55+ MPH there was still a bit of a vibration in no load.

9) I went underneath - the new ujoint's clamp was loose! 3 nuts were barely wrench tight, one was finger tight (Note: remember this symptom I'll explain in a bit).

10) the thing kept loosening, even after a good tightening by the mechanic and some threadlock there was that vibration. He sent me to a Transmission shop. He was sure the rear end had worn out inside.

11) the Tranny shop took one look at the play in the Slip Joint /Constant velocity joint (I've heard it called both) and said the driveshaft must be replaced.

Well I started pricing. Do note Denny's driveshafts they are proud of thier work, a new one would cost around $330. I also called a local driveshaft mechanic. This guy couldn't have been nicer. He said bring it down I'll take a look.

I stopped by yesterday and he put it on a lift. Then he said come here look at this:

1) The driveshaft itself was an inch too short. This hyper extended the slip joint causing the wear.

2) (this was the killer) Ford makes 2 types of U joints. One where the caps in line with each other are the same size, but different size than the other pair; and one where all four caps are the same size. The guy who repaired the shaft and put in a new u joint picked the first type. The end caps were swimming in the yoke with about 1/16 inch play! Because of this - you can't tighten the Ujoint clamp enough and it will loosen itself. Oh - by the way it causes a hell of a racked under no load.

3) He didn't think too much about the double cardain joint rebuild - for the price of that you can buy a new/used/balanced driveshaft.

Well I was lucky, he had a rebuilt shaft of the right size. He sold it to me for only $200 and said "we will put it in, if it helps you buy it, if not no charge I'll put the old one back.

Imediately I noticed the difference - believe it or not rough roads weren't that rough anymore. But when I got to highway speeds that was delightful - I was in 3rd gear at 50mph - I forgot to shift because it was so quiet!. There is absolutely no vibration now. This is a world of difference, I was lucky to meet a nice honest fair mechanic.