Need Help Trany shutters

[Art F250]

Have 02 F250 with the SC installed for 4 years. If i run it hard it shuttered in 3rd gear just after it engaged and and locked the converter. It used to shutter in 1st and 2nd and 3rd befor i had a shift kit installed, Now it only does it in 3rd at 50-55 mph. Two Days ago i reprogramed to the stock chip and now it doesent"t do it that mutch. I recently pulled a trailer that 6600lb and now it does it on a incline and accelerating at the same speed 50-55 I noticed the once the rpm went up 50 rpm and shuttered i let off, I suspect it to be slipping when it is shuttering. I am thinking of going on a 1200 mile journy with the trailer.
Is this normal or is the trany on the way out?


Thanks
Art

[Art F250]

Had the trany rebuilt and a new tourque converter 3 plate. Now works great no more slipage. Work done buy The Transmission Clinic in Newmaket Ont. Great price too 1500. Made in usa parts. The parts that were worn is the clutch pack and the pack seals.