A trick I've used more than once in my life is to add a can of ATF to it, drive it for a day or two, then give it a complete oilchange. ATF is very high in detergents and might do the trick. Seafoam also comes to mind. I'll let some of the other oldhands chime in on this as views will vary.
Your truck may also have worn main bearings. My old '86 Ranger 2.9 had that problem and after doing the ATF with no results and having low oilpressure [my truck was gauged] I resorted to STP...3 cans in the summer and one in the winter. That did the trick for several years, including a trip to Houston and back.
Note!!!
I DO NOT recommend any oil additive [STP, Lucas, Slick50, ect.] in a healthy engine.