I have a pulsing vibration while coasting in nuetral. Starts out as a slow pulse around 50 mph and gets gradualy faster until it is a constant vibration around 65. Started doing this after I put another complete rear axle in it, so that could be the problem, but I'm also wondering if I might have bent the drive shaft when I broke the pinion shaft on my old axle. When that broke I'm sure the drive shaft slapped the ground a few times before it came to a stop. I was under the truck today taking a look, and there seems to be a small amount of play at the t-case output shaft, don't know if that is related, just throwing out some ideas.

 

My old '78 had that problem after I replaced the clutch (and by extension detached the driveshaft). Despite it being just fine before the trans removal, having the u-joints replaced and the driveshaft balanced fixed it completely...until I had to replace the throwout bearing, and it started doing it again. But anyway, I'd bet you almost anything it's the driveshaft being out of balance. A dent would probably really screw it up, mine was visibly fine but it still vibrated at high speed and REALLY bad when coasting.

 

I would jack the back end up and block the from tires.

then just start it, put i in gear and see if the drive shaft has wow'e or isn't straight.

4lo and first, works best.

Flash.