But I have a "Slantmobile", so there was support from the friendly and very helpful people of the GMC Motorhome Forum:
"Maybe your half-shaft bolts backed out / broke off. You may get lucky!"
---------------------------------------------------------------------------------
"Since your speedometer works, things are still spinning inside the transmission. The problem is downstream from the speedometer gear. It could be the final drive."
---------------------------------------------------------------------------------
"First question I have is, does it roll in park??
Now it is common for gmc’s to have the cv flange bolts come loose. Usually the driver side. They are easy to visually inspect.
Second item on driver side, the cv flange has 1 bolt in the center holding the flange to the final drive. The flange can move outward. That is hard to see, because it doesnt have to move outboard much. If that is the case simply have to remove cv. Push flange back in and reinstall that bolt(with locktite).
I believe both those scenarios the gmc rolls away in park."
------------------------------------------------------------------------------------
And finally:
"Most likely bolts came out. Take 3 out of the other side to replace. Then ease it to repair site on it's own power. Get and install new bolts."
"Several people have already suggested this and you can easily look to see if the 6 bolts are present or missing. This is a very common failure. I have repaired a total of 4 coaches on the road for this problem. Crawl under the coach and see if they are there or not on both sides. The left side is usually the one that looses the bolts."
-----------------------------------------------------------------------------------------
So I went to check the bolts.
Missing bolts R. Forum.jpg [ 106.47 KiB | Viewed 2908 times ]
Unbelievable! 6 bolts were gone, as they had told me. Driver' s side. I don' t know what I would have done without the help from the GMC Motorhome Forum!