S10 bucking problems

Everyone, I need some help!
I've been having problems with my 1999 S10 4X4, 4.3L. It started bucking at highway speeds, it seems to be fine during accerleration and idle. I have replaced the plugs and filters but its getting worse. The SES light does not come on when I start the truck. I took it to a mechanic he tried to hook up his scanner but it wouldn't communicate with the truck computer, we thought that the computer was dead (this is under warranty) so I took it to the dealership, they charged me $70 and told me nothing is wrong with my truck. Since then I have checked the plug wires and alternator both seem to be fine. I'm guessing its an emissions problem but the computer doesn't show any codes, but then the SES light doesn't ever come on either.
Any help???
Thanks all!

my first thought would be plug wires, they cause so many problems it's not even funny....i would say it's an ignition problem either way, although a plugged converter can cause probs, but usually all the time, not just highway speeds.

Well the converter had gone out on the truck recently so I just got that fixed about a month or two ago but that hasn't fixed the problem. I'm going to replace the plug wires, cap and rotor tonight and see what happens.
Thanks for the help!