Valve covers are next to nothing to replace. Just make sure you do NOT get the cork one. I've had more trouble out of those.
Check the oil and spark plugs. If the oil is milky chances are you have a blown head gasket. Also, check your spark plugs. If they look like they have been washed (green tint I think, someone may correct me if I am wrong), you are getting coolant into the combustion chamber.
IF it is the head gasket and you decide to do it yourself, it's not too bad. I did one in an afternoon. But I had a spare head that already had some work done it (curse/blessing of having too many spares). Get a FSM or a Haynes/Chilton manual. When you disconnect everything make sure you label it, especially if you haven't one one yourself. Basic hand tools are needed with the exception of a good torque wrench. If you have friend that can help all the better. These 4.0 heads aren't very light and are cumbersome to deal with alone.
If you decide to have a shop do it, make sure it is a shop you trust!! There are little things that add up when a shop does a head gasket on a 4.0. Labor, sending the head out to be checked, assorted gaskets, fuel injector O-rings, etc, etc.
Right now you are still looking for the cause of your problems. If you can, check everything over real good.
Good luck (again)
chris