I have a 1998 k1500 with the 5.7 vortec that shifts normal most the time. The issue I'm having only surfaces when I get on it. My engine revs to over 5k rpm and does not shift without letting off the throttle. It does this every time I floor it to pass another vehicle but if I'm driving and acelerating at a slower rate it shifts fine. The motor and transmission is very strong, suggestions on what could cause this?

 

sounds like a bad vacuum modulator valve on transmission.

 

That is located internally or externally?

 

Update*

Now it shifts a little over 4500 rpm.

 

She wouldn't have a vacuum line to her tranny, she's got the same one you and I have, just a bit different, but all electronic. Try replacing the Vehicle Speed Sensor, I've been doing ALOT of reading on the C/K series, especially the Tranny/ T-Case part of it. Also check the Throttle Position Sensor.

 

located externally, looks like a little can with vacuum line attatched. other possibility is throttle position sensor- unsure of location , although its located externally as well