I have been fighting a similar problem with a 2002 Rodeo for about a month. This is with the stock tires and rims. I get the same vibration you describe, begining at about 65 mph. It began when I had my tires rotated for the first time. Balancing all 4 tires did not cure the problem. Replacing the weak stock shocks with Monroe-Matic Plus shocks helped a little, but not much. At times, for no aparrent reason, it smooths out for a short while. The dealer blames the tires. The tire shop says there is no problem with the tires or rims. Oh, joy.... I guess the next step will be to swap each tire/rim with the spare, to see if the problem is with one tire/rim, or something else.

We are not the first to have a problem like this on an Isuzu. I have read threads over on Club VMag on the same topic.

Go here:
http://club.vmag.com/ubb/Forum6/HTML/000945.html

And here:
http://club.vmag.com/ubb/Forum6/HTML/000962.html

Makes me wonder if they do "on the vehicle" balancing or some such at the factory.

John_Galt