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Transmission shifts into reverse ok and seems ok when cold but after driving a while and put into reverse it vibrates the whole truck when you actually start moving. Any ideas what it could be causing it to act this way.
I had a C4 that was doing this & it turned out one of the pinions was starting to eat it's self. I hope Vipers adjustments work for you - if not then you may need a rebuild.
if adjusting the band doesn't work before dropping the trans I would check the reverse servo. this can be pulled while the trans is in the truck but there is a special proceedure to do it.
Outside the transmission...
In reverse the pinion on the rear axle moves down. This could be enough to put the U-joint into a bind. Easy check, remove the rear driveshaft and back up while in FWD (Front Wheel Drive). If it goes away it is a driveline issue, if not then transmission.
Wrong!
That additive is for TCCs where MERCON-V is called for. Adding this to a C-4 is a mistake. The chemical was developed for late model transmissions like the AODE/4R70W and will not cure the root cause. It could possibly worsen the condition of your C-4.
Do like Viper says... and I would add to his advice, also check the rear servo piston to make certain the rubber is still soft and pliable.