mud is an entirely different animal than rocks etc.
you'll be spinning the tires in order to clean them, thus carrying speed/momentum, so, crawling up an obstacle is a non issue.
given you'll be spinning the tires, if the front is locked, and the tires are spinning, the rig will want to go dead straight.
lock the rear;
1. you will have the ability to turn the rig while under power.
2. you will already have momentum, so crawling up something is moot.
3. the locked rear will allow you to pivot the rig under power.
lock the front;
1. under power, while spinning, the rig will want to go dead straight.
2. you will need to kill momentum to turn... in mud... you decide.
3. if you try to pivot the rig, you will go straight.
If you wheeled rocks and ledges, I would advise a front locker...
You wheel mud, where wheel speed, momentum, steerability and ability to pivot rules.