It sounds like the shop that did your transmission doesn't have the proper settings for the shift points and stall speeds. Also, when you have higher output engines like ours, the transmissions should be set up way differently that required for a stock set up - if that makes sense. My transmission was done by ATO in Sacramento, Calif and I was told that in it's current form, it could handle 650 to 700 HP although I'm nowhere near that amount. It shifts like a car unloaded and is very precise towing. My excursion weighs nearly 9000 lbs and sometimes I tow a boat that weighs 85oo lbs and the transmission performs perfectly with or without load. Also, Jim at ATO offers the extreme rebuild that handles way more than the 700 that mine can handle.
I think you need to return the truck to the shop and have them take another look at their settings. It won't last long set up as it is now.