oscillation vibration?

[swamp rat]

I used to own a 76 Chev 4X4, it was full time when I bought it, I installed the conversion kit and put locking hubs on it.

This truck had this oscillation vibration at certian speeds, I think it was it was only in a 5 MPH range around 60mph, anything above and it would stop, anything below and it wouldn't do it ether. it did it before and after my mod.

The vibraton was kinda like 4 seconds on and 4 seconds off, just a steady rhythm intil I changed my speed.

I know what a tire feels like, its a steady vibration at certian speeds, so I allways figured it was in the drive train somwhere but never figured it out.

Anybody experiance this and know what causes it??