i have a moser 9" with the alum thru bolted center and 3.89's my question is does this swap require a diff drive shaft? i have my stock one installed and i have a vibration during hard acceleration and highway speeds! i have new u joints. everything is tight except at the yoke where the driveshaft enters the trany you can grab it and wigle it around about a 1/16 of an inch, just enough that you can see a gap in the seal. i think the ds is to short now it has about 1.5 inches showing with weight on the axle

 

im using a stock length shaft with the same rear and i have no issues, but im running a tko600 manual trans, but its supposed to be the same length as a 700r4/t5

If the rear is centered front to back in the car, and you have 1.5" of the yoke sticking out out of the trans, you need a new driveshaft.

 

If you are using the same drive shaft that you had with the 7.5 then it is too short. Which yoke do you have on the 9 inch? If it's a truck yoke then it will be much shorter than the 7.5 was. The car yokes are a little longer, but still not quite long enough.

 

im not sure if its car or truck. its a moser thru bolted aluminum center section
though. i did reuse the drive shaft

 

 

Try unbolting the driveshaft from the rear and sliding the yoke in more and see if you still have that problem with the wiggle. Mine is out about a inch and i have no wiggle, it's good and tight.

 

Proper length is to slide the yoke into the tranny all the way then pull it back out 1". If the u-joint isn't in the diff yoke at that point then the driveshaft is too short or you need a longer diff yoke.

 

If you are making enough power to need to upgrade rear ends, then you should be upgrading the driveshaft anyway.

 

if you use the factory driveshaft you need to change the u-joint to a 1350 on one side and a 1310 on the other you can use the same drive shaft i have the same rearend and gears

 

Third gen yokes are not 1310. They're called 3R. Close but not exactly the same.

 

didn't know that but you can't a 1350 to 3r ujoint but they do make the 1310 to 1350 conversion joint

 

That is the short truck yoke, and it's probably a 1350. I recommend that you get a new driveshaft made with 1350 yokes on both ends. Contact the drive shaft manufacturer that you are going to use and ask them how they want the mesurements made. Not all manufacturers use the same measurements. Inland Empire Drive Line is a good one. iedls.com