Filmhog

I have 2002 2500. I have 120K on the engine and tranny. It runs gread with the edge ez box.
About 30K ago, I would get a slight slipping or vibration when accelerating and hit the 1800 rpm mark. I back off slightly and go through that zone to 2000rpm. It smoothed out. Now the shuddering is more pronounced and easier to instigate at the 1800 rmp point. when towing I just take it out of OD and that moves me through the shuddering point more quickly. It is most pronounce when towing but will do it without a trailer when getting on the freeway. (spilling coffee from the shuddering vibration all over....grrr.)
Took it to a good and recommended tranny house here in Reno and they had no codes on the tranny and recommended I drive it until I get a code or something breaks. I depend on my truck for work and such and take good care of it so that is not an option. neither is $3K for a new tranny if it blows. I asked if the tranny was "slipping" and he said no, the computer would announce that. Have we gotten to the point where a computer replaces common sense?
Can anyone tell me what is going on with this tranny (or drive train, or front axle????) Thanks for any help. I've lurked here as amember for a while.
Jan