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.