diesel 6.7 engine wont shift into 4 gear ,rebuilt tran ,new oil on 1996 GMC Vandura 3500

please help ,i try a lot of mechanics, i don't what to give up !!!!!!

Asked by for the 1996 GMC Vandura 3500
did you return to the re builder??? it should have warranty.
I live outside Phila. If you dont get any results, you can go to a place that has done all my units in Phila. It is called Xpert Transmissions and the guys name is Mark and is a first class place.

can u send me the phone number please,,,