srumble only in OD

My 98 GMC has developed a rather pecular issue that maybe some of you can help with.
Truck is a K-1500 4x4 with pushbutton 4wd.
While driving in OD around 45-65 MPH, when applying a very slight
acceleration load, the truck 'stumbles'. Kind of like a missfire, but it doesn't set any codes. Plugs are new, gapped correctly and it runs fine the rest of the time. If I drop down out of ID, it doesn;t miss at all, only in OD. It's not a problem yet, but it sure is annoying.
Any ideas where to start looking before I head to someone with a OBDII computer that can monitor it under load. My reader only reads codes, and as I said, nothing shows up.

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