Engine/Transmission (1998.5 - 2002) rough when lugging

[DEspen]

Why does my 99, 2wd, 5 speed, with Banks Git-Kit run rough when it's lugging with a load? Usual load is 2k camper and 21 ft boat. Down shift will sometimes help. Seems to have gotten worse with install of South Bend 13" flywheel and clutch. After clutch install a new strange vibration comes in at 70. Goes away if you lift off the gas but comes back even if maintaining speed downhill. That's with camper only as load. My altitude is 5k to 7k and transfer pump is stock. Pressure after filter is 13 light load and drops to 10 or 11 when lugging. Changed all driveshaft u-joints but it didn't help. Any ideas? DEDW16