Going Into Reverse

[Storsy04]

When im in park and put my 04 gmc sierra into reverse i hear a clunking noise and then it engages into reverse sometime it does it and sometimes it does not is this i serious problem or could lead to serious problems? i just hit 94,000 miles with it and i am planning to get my tranny flushed and refilled befor winter

[Glenn Black]

my truck did the same it turned out to be the U joint at the rear diff. Although it could be the diff too. It's easy to check the U joint by twisting the drive shaft while the truck is on level ground in park ( you will be able to see movement and hear a slight ticking)