Hey, I drive a 97'f-150 2wd ex cab, and im having some issues. Between about 30-40 mph, my truck feels like im driving over rumble strips. does is again faintly at about 65 mph. I dont think its my transmission, i checked by putting it in 2nd and seeing if it was a smooth ride... not at all! same issue. I changed my rear diff fluid, helped a little for about a day, but still having probs. Any thoughts?

 

Changed the fluid and it helped? Any noise? Was the fluid low? Fluid dirty, metal flakes, shavings? Does the pinion have any side play, in our out play?

 

You could check the u joints.

 

Mine does the same thing and the torque converter is bad but mine is 4 wheel drive. Try driving down the expressway doing about 70 and engage a downshift and see if it does the same thing.

 

changing the fluid helped yes, but only for a little. fluid was bad, so could im guessing there could still be a good amount of sludge in there?. No play with the pinion tho. Should i Flush it again???

 

Normally I would say u-joints or torque converter as possible causes, but since you changed the fluid the problem went away, you're in the right area. Did the problem (rumble strip) go away totally? How many miles?

 

Actually torque converter was ruled out do to the fact in your original statement that you put in 2nd and still have the same result at the same speed, the torque converter spins at a different speed which would have put your vibration at a different speed.

 

the problem only lessened. i noticed a big difference at first, but now it it worse then it was before. Im going to change the diff fluid once more. i used 85w-90 gear oil. What oil should i use?

 

When you change your fluid take a look at the ring and pinion, make sure your ring gear has no side movement.

 

Im gonna try and do some more work on it this week. I picked up a rear at crazy rays junk yard, im just gonna replace the whole thing... i know the problem is in the rear now. I will look at ring and pinion again and let you know if your right or not. Once the rear is in, im gonna be selling it. http://westmd.craigslist.org/cto/1580585963.html
I dont know if im aloud to post this on here... so sorry if im not. But i will let you all know how it turns out and what the exact problem was hopefully by thursday. Thanks for all your thoughts and time.