Have a 2005 GMC Yukon, 5.3 all wheel drive. Wife was driving it and suddenly it wouldn't go, she gave it more gas and it clunked and started to go but was now making noise. She pulled off the road and got a tow home. Turns out, now it's not holding in park, when shifted to reverse with brakes being held it makes a super loud grinding noise. Shift to drive with brakes applied and and there is no noise until you let off the brakes and the tires start to move. I'm suspecting a bad transfer case but do these symptoms match up? Also, when I went to drain the Tcase, there was way too much fluid in there, so it must also have a bad rear seal. Could this have contributed to failure? Thanks for the help.
Do you still have propulsion when you switch to D?
Pobably not, otherwise your wife wouldn't have the need to be towed.
Foe me it sounds like the clutch pack for the reverse gear has broken down and caused damage to the transmission.
But of course it can't be ruled out that something has broken down in the transfer gearbox, which only makes noise when turning backwards might.
Does the GMC have permanent all-wheel drive or switchable four-wheel drive?
The first thing I would do, is lay under the car and carefully open the oil drain plug from the transfer gearbox and drain some oil and see if there are any chips there. When was the automatic transmission fluid last changed?
Take the dipstick out and smell it to see if it smells burnt.