This sounds like the oil pressure relief valve (which is usually located in or next to the oil pump) is sticking open; or the relief valve spring is broken.
Have you checked the oil level? Dodge trucks usually take significantly more oil than most passenger cars. Sometimes mechanics do not fill the crankcase to the proper level. This is particularly common if a mechanic is not familiar with a particular vehicle. This could also happen if too light a viscosity of oil was used.