Thanks for the updates everyone.
I've spent some time under the truck the past couple days - my driveshaft seems to have several large weights on both ends. I measured runout of the driveshaft and it was 0.035" nearest the transmission, so it seems they correct the runout with weights to get everything back in balance. Makes sense I guess. The transmission mount/dampener there seems really soft (I could shake the driveshaft/trans with my hand quite noticeably) so I can see how a slight balance issue might shake at high speeds.
For fun, I strapped a couple of washers to the driveshaft to see if I could affect the shake - well sure enough it seems to really have calmed down to a point I barely notice it, at least in my quick test drive this evening.
I wont have a chance to try anything else over the next week or so as I am heading out of town, but maybe this info helps.