hey yall new to the board,
got an 04 4dr 4x4 84k
recently took her on the interstate for the first time and noticed a pretty bad shaking in the seat, not wheel, above 60 mph; it only gets worse with speed
got tires rotated and balanced. filled front diff fluid, transfer case fluid, and rear diff. no luck still. thinking its not ball joints or idler, shake only in seat; steering wheel is smooth; def not in 4wd, (i use my vise grips when i need that)
thinking u joint maybe? rear out of alignment? anythings helpful

 

Could be a bunch of different things. Bent axle, bad universals in the rear driveshaft, out of balance driveshaft, etc.

For the bent axle theory, jack up the back end of the truck and spin the tire. If it is really bad, you should be able to see it. Otherwise, you may need to find someone with a dial indicator, arm, and magnetic base. Put the base onto the axle tube and the dial indicator onto the inside of the rim. Rotate the tire and watch the reading. There should not be much, if any, movement indicated.

Bad universals can be checked by either pulling on the driveshaft or just removing it and doing some close inspection.

Out of balance driveshaft should not occur unless the weight has been knocked off or otherwise removed/damaged. A visual inspection of the driveshaft should reveal this.

 

i did that visual test and the driveshaft seemed to be fine. there are a few weights on the driveshaft and it doesnt look like any have been removed or come off
im thinkin a u joint because when i put it in drive i and it engages i hear a "clunk". the old u joints are pretty rusty too. also when i hit some bumps at low speed (35 down) i hear a faint squeak, kind of rattle noise. not ball joints or bushings, maybe a bad shock though

 

sounds like u joints to me. or a loose pinion yolk. check your transfer case mounts going to the crossmember also