1996 Yukon/RPM fluctuation

I am experiencing RPM fluctuations while driving on the freeway. It is very noticeable when driving between 55 & 65 miles per hour. It happens more frequently when in OD but will occasionally do it while out of OD. The RPM's will dip 3-4 grand and then grab again. I haven't taken the rig into the tranny shop yet as I wanted to get some idea as to what it may be before I walk in and get bent over. The vehicle only has 118,000 miles and I've never towed anything with it. I was wondering if it may be a vaccuum issue or possibly an injector problem but I am the farthest thing from a mechanic. Any troubleshooting ideas would be greatly appreciated.

Scott/Portland, OR.

A full scan would be the normal starting point unless you can manually trace this to a misfire issue.