if my mind is working correctly, when in 2wd low, the front case is shifted into low, which pushes the shift rail to the rear, now your rear case is in 2wd so your lo hi shift rail is forward, if the shift fork shaft is not cut off properly it will touch and cause what you are discribing.
if im wrong Mike, Marlin just shoot me 
Thats it
The rear case is probably an 85-88 w56 top shift case and if it is then it has an extra little half inch of the shift fork that needs to be cut off... How do I know? I didnt follow the instructions correctly on setting up the dual cases cause it says to cutt that off (only problem is it is at the very bottom of the instructions to set them up so i totally went over that...)
How to fix the shifters... Pull off the rear case. leave the adapter to the crawl box and thencut off the extra little thing that sticks out. its like a half inch or so long.
Here ya go:
http://www.marlincrawler.com/htm/transfercase/crawler_installer/index.htmTowards the end it says in the rear case part, 3rd litle paragraph down it says to do that.
"If your rear-most case is out of a 1985 to 1988 top shifting EFI (W56) tranny then you must shorten the driver's side shift rod. This is the shift rod sticking out of the rear most case when you shift into 4hi that has the flat side on top. You must trim 1/2" off of the shift rod to avoid jamming between the two cases. See marlin's instructions that came with the adapter."
