I have a 1995 F-150 2wd. Under normal acceleration I get a vibration/shimmy usually starts at 44-45 mph and goes away at 50 mph. If I'm getting on it, pedal to the floor, no vibration. I have load-force balanced the tires and they are within specs. U-joints are great, no play or binding. I just replaced the rear drums because a section of the outer lip was rusted off. I thought that may be causing the problem, but it is still there. I feel it in the rear of the truck, I can see the rear of the box moving side to side in my mirrors. Any help would be greatly appreciated.

 

jack rear end then have run in gear then look at driveshaft see if it not bend.

That what I have same problem vibration at 38 to 50 but gone 51 to 65 mph.

Then found when jack rear end and watch it was so bad it not straight it bent.