2000 gmc 2500 4x4

have 2000 gmc 6 liter. trans is shifting very hard after truck has been driven a while. i can stop and shut truck off and restart and it will stop for a while. have had filter and fluid changed, still continues to do same. computer shows code of 1870 or 1800 do not remember which. has anyone had any simular trouble, and if so can anyone advise waht is fix. i have been told that trans needs to be replaced, bad valve body. i even paid a guy 350.00 dollars who said that he replaced bad valve body, but truck still continues to shift hard. can anyone please help. thanks my e-mail is [email protected] thnaks and happy holidays