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This is probably an existing topic somewhere but maybe with a different twist. I have a 68 f100 longed, originally was a 390 3 on the tree. It's now a 5.0 , 5 speed. Basically had the drive shaft shortened to accommodate the 5 speed. The shop that cut it done a great job, I brought it home and it bolted right up, new u joints and the carrier bearing is only a couple years old. I put a New set of tires on it and for the first time since I've owned it I took it out on the open road. Getting a minor to moderate vibration at higher speeds. The drive shaft was not balanced with both sections together, shop only had the section that needed cut. Is this pretty much the problem in the opinion of the experts out there? The front end had new bearings, bushings and shocks about 10 months ago, rear shocks are New.....
^ That could be a possible issue. Would also look into either your tires being improperly balanced or possibly a wheel bearing issue. I would be looking into each of those as your problem more than likely lays there. You had the new tires put on at the same time you had the driveshaft put in? Making major adjustments to the driveshaft can very easily cause issues as it is quite a bit more complicated than people realize. (Such as alignment issues, etc.)
I drove the truck locally for a month or so after installed the drive shaft, had not so good tires on it do I've never had it out on the open road so I never noticed any issues. The day Ieft the tire shop I hit interstate 40 out if excitement to for once hit 5th gear and ride a little then around 65 or 70 I felt the vibration. It's not just in the steering wheel or seat. I rechecked the front for any bearing slop and everything else and all that is new/good.
I'd take it back to the tire shop an tell them. But do check your drive line u-joint clocking first.
So many shops don't do a great job of balancing for the upper speed limits.
Look for a slightly bent rim wobbling also.
Even some shop balancer get out of balance!
Orich
I can confirm what orich said. Worked at a tire shop for a while as a previous job. Had a day where out balancer went out of wack and had a whole days worth of customers come back with there tires off 5 oz. +
Make sure your front 1/2 driveshaft is level with your transplanted 5 speed as shown in Hio's pic. The answer to your vibration may only be a shim or 2 at the trans mount or carrier bearing.