It seems I have more serious transmission problems to worry about. I took off the PTO cover on the back of the transfer case and pulled the gear off the mainshaft. The splines on the shaft and in the gear are pretty much gone - it may be time to drop the gearbox and transfer box, strip and rebuild both. At the very least I need a new mainshaft and mainshaft gear. I'm sure I can sort any other problems out during the rebuild.

Not what I was hoping to find!