Not just older 4X4.....We call it "death wobbles" in lifted jeep world. It goes from a small shimmy to an all out uncontrollable shake that threatens to rip the vehicle apart. Only way to get it to stop is slam the brakes on and get the speed off as fast as possible.
This problem he is describing here is definately drive train related, everything recommended to check above is valid. Just as in jeep world a systematic check piece by piece will generally allow you to find the issues and correct them.
Here is a good read. It is talking about jeeps but it applies to all.
http://www.dirtroadmagazine.com/articleb.htm
