1999 Dodge Truck oil pump

Ok I have a 99 dodge ram 1500 4wd. My problem is As I am driving it my oil pressure drops to zero. If I tap the gas it comes back up to normal. Then drops off again. When I saw this I drove it about 6 miles home and haven't drove it since. Is this the oil pump or sending unit. Both are fairly cheap parts but where is the sending unit and do I have to lift the motor to get to the oil pump. Also the truck will not heat up the temp gauge doesnt even register and never blows out heat. Are these problems related? The engine sounds great no banging or tapping at all.