Any grinding during shifting or while in those 2 gears?
Any new vibrations while in those gears?
Probably the first step would be to check the clutch fluid for correct level, then follow the hydraulic line down to the trans and look for fluid leaks, look for visible droplets and moist spots that indicate fluid was there.
Check the bottom of the trans below the bellhousing also.
Next pull up the shift boot, make sure the boot or inner liner arent causing issues with shifting.
Then remove the shift lever and check for unusual wear or visible damage to the mechanism in the top of the trans.
Finally check the bottom of the trans for signs of gear oil leaking, if its 2wd look under the truck, on the driveshaft, and at the rear seal for signs of leakage, 4wd look at the gasket wher the trans and t case mate. Check the trans vent at the top and the shift tower for signs of leakage.
Check the trans lube for proper level, top fill bolt on the side of the trans should have lube even with the bottom of the threads.