Another tranny question 98 k1500

I have a 98 k1500 z71 with automatic transmission, I replaced the transmission with a rebuilt transmission and replaced it by the book, not taking any shortcuts. I followed the break in recommended by the place i bought the transmission from. All was well and after break in i pulled my boat to the lake quite often. And when i pull the boat or something heavy i pull it in 3rd because i dont have an O/D switch. The truck has been sitting because about 4-5 months after i bought the transmission, it out of nowhere started acting up. Anytime you are cruising or under load(even if its a small one) and the truck shifts from 3rd to overdrive it shifts unbelievably hard. Im not talking about a regular hard shift, i mean it shifts so hard it actually makes the truck and all people inside of it move. When i drive with the truck in 3rd " |3| 2 1 " it doesnt happen. Its just with 4th/overdrive gear that this happens. Any help would be much appreciated. something else to add. Anything over 65mph i have a rediculious vibration, i know for a fact its not U-joints or tires. U joints are replaced and new, tires are new and balanced.

Needs to be put on a transmission/engine capables scanner and read out shift and slip data and codes.
Get codes and post back code no.