Last week I suddenly got a vibration while on the highway and at first it felt like the motor lost a cylinder and I was only running on 5. I changed lanes and everything was fine for about 30 minutes of highway driving. Then as I got off the highway it started to shake and I could feel the shake/pulsation while braking. I tried braking again and it was fine, no pulsing or shaking. Then as I pulled away from a light it started to shake/pulsate violently and I had to pull off the road. I made it home and I thought it might be the master cylinder acting up. So I put in a rebuilt master and sure enough I got the vibration again. 95% of the time it's fine and brakes good and straight so I don't think it's brake or rotor related now. But while it started to shake today I pushed in the clutch and no difference. I shifted into neutral and coasted and it was still shaking. So I'm thinking either suspension or driveline. The shocks look good and feel pretty stiff still. The driveshaft looked fine and felt like it was in place good and firm but I didn't get a chance to jack it up and rotate it. It will just suddenly decide it wants to vibrate and shake and I have no idea what the real problem is. Any thoughts?

check your drive line bolts.
& balance of your tires
check the hub lockers

I just went out and jacked it up to rotate the driveshaft to look for anything odd. No sign of cracks or anything. I decided to grease the u-joints and driveshaft while I was under it and the driveshaft was taking a lot of grease, the u-joints took a little but I don't think they were too bad. I'll have to wait until tomorrow to see if the grease helped. Shocks look/feel good, springs look good, torsion bars/mounts look fine. Is it possible the rear proportioning valve is going bad and causing the rear brakes to partially apply by themself?

I didn't think that was how the LSPV worked...AFAIK, it just changes the amount of pressure the rear brakes get in relation to how much it's open......but to make sure, you can just unbolt it from your axle and run with it tied up against the frame, lots of guys do this on purpose.

After the vibration happends, stop and feel the wheels/hubs. If one of them is really hot, you could have some brake drag going on.

Well, the grease didn't make a difference. But I think it's my rear starting to go. I need to check the fluid level tonight but it might be too late already. If the fluid is low, what are the chances adding fluid will make the problem go away? I think it's locking/unlocking the rear.

The more I think about it I've noticed that my parking brake isn't holding as well lately and sometimes the truck almost feels "loose" on the road. I definitely suspect rear end problems but don't know exactly what it might be.

Where does the vibration come from, the steering wheel, the seat, or from behind you? The steering wheel would be an indication of the tires or front hubs, the seat would be a drive-line issue, and if it feels like its comming from behind you it woul be something in the rear axle. I would think if it is anything with the front susp/tires/hubs, the driveline or teh rear it should happen all of the time, not intermitantly.

A few years ago I my 3.0 would kind of shudder, though never violently enough to make me need to pull over, just the CEl owuld come on and then go off. The ECU registered it as a distributor code, the dealer diagnised it as a burnt valve. I rebuilt the top end w/o replacing the distributor and haven't had the shudder since.

I think I found the problem. I thought it was the rear so I drained the fluid and changed it. It wasn't really too bad at all and then I checked the previous owner's notes and he had done it twice before so it wasn't a rear fluid problem. I jacked the rear up and just like last night, the passenger wheel wouldn't turn! Well, the parking brake was frozen on the passenger side. I removed the wheel and started to spray the lever with PB Blaster and it would move a little but the spring couldn't pull it back. I pulled the drum off and checked the moving parts on the wheel side and they all moved freely. So it was definitely at the lever hinge point that it was sticking badly. I was able to remove the bolts holding the lever on and put it in the vice and hit it with the heat gun and PB Blaster and kept hitting it back and forth with a hammer. The heat allowed the fluid to break free most of the rust and it finally came loose! Wow, it was really rusty and that lever has a rubber boot to protect it from moisture. I haven't driven it yet(I worked on it from 5-9 tonight after work on a wet driveway) but I'm sure that was the problem.