We have a male shepard/golden retriever mix that we got from the Humane Society. They estimated it's age at 2 years. We had him nuetered at that time. It's been almost a year later and he has recently started urinating in the house. We have two dogs. The other is a 3 year old male full breed german shepard that we've had since a pup. The GSD does not have this urination problem.

We keep the dogs outdoor during the day and bring them inside the house at night (never unsupervised). We let them out frequently when they are inside. He goes 8 to 10 hours through the night without problems and only urinates after being inside for a short time (1 to 3 hours). He picks a different spot each time (always a leg lifter) - no excitement going on around him either. I have the Leerburg Basic Training Video, a prong collar, and a remote shocking collar. He definitely has basic obedience down.
Why is he suddenly urinating and, more importantly,what can I do to stop it? The first few times, I didn't actually catch him in the act, but I did hear him a few times (didn't really think much until I later saw what happened). I put his face up to it, said "bad" (he knows this word), and gave him a good hard slap on the rump. Finally, he did it right in front of me. This time I said "no...bad" and slap him a few times (harder than previously), then sent him outside for a few hours.

What else can I do?

Before you ring his neck again please have a urine sample checked at the vet. He could have a low grade infection or something brewing. Even though he has been neutered there could be a female in heat somewhere around that causes him to still remember what "that" was all about. He may be trying to become "TOP DOG" if the GSD is right now....trying to climb up the ladder as it were.
But please, don't punish him if you don't catch him doing it...the guy has no clue what you are doing...I don't care how smart he may be.
First things first...get his urine checked...it may just be a simple infection!!