You can install certan gearing until you jump too large/small, and L/S will still work, with the rite gears. i went from 3.65(?) to 4.11's, and it didnt need replacing, but a 4.56, would require a bigger carrier.
If you are going to wheel it, go with a locker up front, (4.11's are good gears), and keep the L/S in the rear. Did this in my 67' Bronco and worked great(eaton E locker)! Love the D44 in that thing, its a high pinion! And the NP435 trani is bulletproof, i would switch.
Look for leaks at hoses, for P/S, bubbles in the pump, Pump might be bad, boxes are very strong on those!
This is strictly for wheeling!
A killer site is Broncograveyard.com They have alot to offer to the 78-79 Broncos!
Good luck, very fun rig!