How does the clutch pedal feel?? Sounds like the clutch may not be releasing fully. Is it only happening when the truck is sitting still ie:going into first gear to start off and seems to shift fine while driving?? Just a thought. Check for a leaky slave cylinder or air in the hydraulic...
Best solution I've found for extra weight capability without sacrificing ride is a product called "road master active suspension". I've installed a few dozen of these and never had a complaint. Never had a set fail, hardly affect unloaded ride quality/ride height and are completely maintenance...