How did it drive in the 1/4 mile to your driveway? Any weird noises? The jumping out of gear & grinding noises does sound more likely to be a transfer case issue than Allison. I haven't had any similar issues with MK48, so I don't have any great solutions to offer.
Have you tried putting it in low range? That would tell you if it is trans or transfer case
You might want to see if you can see where the transfer case shifts underneath and see if you can see if shift linkage is moving appropriately. I have not looked at MK48's mechanism, but we ran a lot of big trucks with power dividers similar to PTO which usually shifted with a small air cylinder which can pretty easily get gummed up.
I think the MK48 transfer cases are different than Hemtt. Likewise, I believe the trans are very similar, but have some differences.
I'll take a quick look at TM & post again if I see anything that might be helpful.
My mk48 arrived today,after the trucker unloaded it.I drove it about a quarter mile to my driveway,about 200 feet up my driveway it jumped out of rear.I drifted back down the hill and checked all the switches and transfer case lever.It made a loud grinding noise when put in forward gear,it seemed to want to back up.I called the trucker who worked for the dealer I bought the truck from.He dint know what to do.I then tried backing up my driveway and it went a little ways and jumped out of gear and started making a loud grinding noise.I called the owner of the dealership and he said he would send the trucker back to get it.Now I dont know what to do.If they take the truck back they wont send me another truck out delivered for the same price.They are a 1000 miles away.It sounds like the transfer case went out.$30k is a lot to spend and not have a running truck.the truck is in beautiful condition with only 185 miles.Any suggestions??