Transmission slip? 00' Excursion

[Aquaticedge]

I've noticed something odd, and I don't know really if it's a slip or just normal. on my excursion I've noticed a sort of Shimmy at about 45-50MPH. It's best described as the transmission cant decide what Gear to use and hops between the 2... but it's not like a normal shift, its more subtle, and I want to head off any problems while they remain small... Any ideas? It's a 7.3L Power Stroke 190,000 Miles