I did a search for this and didn't come up with anything so thought I would post in case someone runs into this issue. '99 310E 4wd, had the machine on a job and noticed it was very hard to turn. Using the diff lock foot or 4wd switch didn't help. Got it back home and went through the service manual, you will need one of these! There are 3 solenoids on the transmission that control the park brake, diff lock and 4wd. All are located on the machine's exhaust side, pull the black hyd tank cover off for a better view. You can pull each of these out and test by holding the solenoid and having someone play with each switch, make sure the plunger is working. Be very careful when reinstalling them, the cap holding them on is plastic and will snap very easy, use 5ft lbs of torque. I ended up finding the diff lock plunger stuck and replaced it, local dealer wanted $680 for the solenoid but found a new oem unit online for $540. All 3 are the same so you could swap each depending on what issue you are having, might save you lots of money vs throwing parts at it.
Thanks for the info.
So the moving part of the dif lock solenoid was stuck?
Yes, there is a plunger on the end that the magnetic field pulls in to let fluid pass to that channel.
Interesting that it stuck in the energized position. I have the same backhoe and I have bench tested all those solenoids. The return spring seems strong. That said, one grain of sand could probably stick it.