Welcome to MTF Ron! Lots of info here and great member to help and share with, enjoy the site! Was there any strange noise before this happened? Sounds to me like one of the gears on an axle end has stripped and until you hit the diff lock the differential is just spinning freeley, hitting the lock engages the one that still has teeth on the gear.
get in some loose gravel or dirt and strap the machine to something, then hit the diff lock and see if both rear tires spin. They should both spin, if only one then the side not spinning is the side that needs to be looked at.
Depending on what year your 425 is, the most likely problem is the spider gears have stripped. I had the same issue on my 1994 vintage 445. The early 4x5 tractors has a set of spider gears consisting of only two gears. When my tractor had the same problem, I removed the Trans-axle and opened it up to find those gears stripped out when I went to the dealer to get the parts I found that they had been superseded by a four gear set. This is a lot stronger and has not given me any problems since. You will need the manual to fix this and if you can a print out of the parts breakdown will also help. I am on the road so I don't have access to the K-92 manual for the X7xx series, but you should be able to find it on the net some where. It is very similar to the K-91which came in the 4x5 tractors. The JD Service manual is the best though if you can get it. Be careful removing the Axle mounting bolts from the frame, my tractor was so corroded on one axle that the bolts snapped off and my son had to use his mill to remove the steel boltd fron that axle housing or I would have had to order a new part. this all was repaired in the late 1990sjust after my warranty had expired, probably 96-98 time frame.