I've got a terrible vibration as well... It all started when years after my 4" procomp lift ( IFS ) , I broke my rear springs, then bought some 1.5" new ones from NWOR, coupled with having to crank my swayaway torsions 1.5", gives me around 5.5" of Lift........well it took about 6 months to show up, but that angle on the rear drive shaft, was obviously too much.....it backed the T-case output shaft nut off, to where it obviously had a real bad vibration.....I looked underneath the truck , to see if I needed to perhaps change the front ujoint of the rear shaft, I found I could move my ujoint and driveline about 3 inches up and down ( probably not a good thing ) , anyway, not being a mechanic, I took it to a driveline shop that works on Semi Trucks, and they custom built a Slip yoke drive shaft, with two cv's on the t-case side to split the angle .......unfortuanately, the vibration is still there, usually above 50mph, while on the gas pedal , deaccelerating it goes away. So now getting underneath and feeling the driveshaft and the rear T-case flange, it moves very slightly, but I suspect it's probably the vibration problem. I had the driveline shop assure me that they balanced their shaft on a lathe at high speed, so I figure it must be the output shaft bearing thats bad, Plus it leaks T-case oil, there, too.
So I'm thinking , I could go to Fresno, ( I'm the SF Bay Area ) , buy Marlin's single case 4.70 and have them install it ! .......hopefully that takes care of the problem !!! I know you mechanic guys would probably rebuild it, but again I'm no mechanic, and since in the 5 years I've owned the truck , I have done many mods, or had them done, including the gears and the tranny, that now the T-case is the " Weakest Link ".....
anyway, what are your thoughts ?