If it is the aluminum 5 speed you need to compleatly dissemble the transmission and replace the rod the shift fork rides on. Or the case is worn out letting the shift fork rod to have some play, regardless you need to take the trans apart to see what the problem is. If it was the old 4 speed you could just remove the shift cover from the top of the transmission assembly and replace it with a rebuilt unit. I would take out the switch and look in there with a flash light and see if you can see if the shift fork rod has a groove worn in it when it'* in reverse.
04 Yukon XL 2500 4x4 8.1
91 K-1500 5.7 RV cam 700-R4 stage 2 shift kit
08 Cadillac SRX 3.6 AWD box stock
49 *****'* CJ3A Jeep
There is no replacement for displacement!