Hey man, sorry to hear that you're having trouble. It sounds like your transmission is slipping as it shifts into reverse. If you haven't yet checked your transmission fluid, you may want to start there. The transmission shifts gears by modulating fluid pressure through an arrangement of valves, so fluid level and condition are important.
If the fluid is within specification, you might want to consider taking the truck to a transmission mechanic for a line pressure test.
I wish I could give you definitive assistance by way of the internet...your best bet might be to bite the bullet and take the truck to a dealer for diagnostics. From what I know, the problem could be one of the sensors that feeds information to the TCM (vehicle speed sensor, PNP switch, etc), or it could be a problem with the fluid management assembly in the transmission (pressure switch 6, reverse clutch solenoid valve, etc). I believe the latter will require you to replace
this part.
Best of luck.
