First, having 4lo but not 4hi with a manual TC is pretty unusual. You might try adjusting the linkage to the shift lever, it may not move quite enough to get into gear.
You might also try checking what's happening by shifting into 4hi and moving the truck a little, then raising one of the front wheels.
If you can rotate the front driveshaft, then the TC isn't in 4wd mode.
And if you can repeat this test, but in 4lo, and you can't rotate the front driveshaft, and adjusting the shift lever linkage doesn't get 4hi working, then something is worn/busted inside the TC.
2004 GMC Sierra 3500 C&C, added 4WD, 6.0L gas, Extended Cab, SLE, Transgo 4L80e-HD2 kit installed
Whee! Dually Club Member #1109