jdk1500

So I recently swapped a dbc 5.3 into my 1998 k1500 with a 4L60E. I used a painless harness (because summit had one in stock and I live about 10 min away). The computer I used was out of a 1999 Camero, but I had it flashed with a 5.3 truck tune. Anyways when I drive the truck, the tranny will not shift on its own. For example, when you put it in drive, it is like the tranny is starting out in 4th gear. Any suggestions as to what could be the problem would be appreciated. Thanks