Last night / today, I put in a PowerTrax No-Slip rear locking differential! First time ever opening the rear, the fluid was changed around 31k miles, I have 55k, so it was about time to change the fluid anyway. The PowerTrax set me back $450, the gear oil about $30.
Earlier this week I also changed the front diff fluid to Lucas 75W140, changed the transfer case to Castrol ATF+4, and pampered the Liberator with a full synthetic oil change. Those fluids was about $75.
Up next I will be putting on a new rear diff cover and re-replacing the oil with Lucas 75W140 - I figured it would be a good idea to let the unit break in a bit and change it fairly early.