'99 Tranny Problems
 

my buddys 99 has a weird problem about shifting repeatedly many times all really fast all at once in between 4th and OD at around 1600-1800 rpms any ideas whats this could be?

Reset the APPS and it will be fine. That's how I fixed mine.

Disconnect negative cables from both batteries
Turn the ignition switch ON
Wait at least 30 minutes (some leave it overnight)
Turn the ignition switch OFF
Connect both neg. batt cables.
Turn the ignition switch ON, but do NOT start
S-L-O-W-L-Y press the accelerator pedal all the way to the floor.
S-L-O-W-L-Y let the pedal come all the way back up.
Start the truck and road-test it. See if the problem went away. If not, there are some more extreme measures,
such as shielding the alternator wire.
Hope you don't have to replace the 400+ dollar APPS sensor.