I had planned on replacing my U-joints and I have been doing some research online (OEM vs. aftermarket, etc). I noticed that RRO offers greaseable U-joints and it appears as though my current U-joints have grease zerks on them.

Did the previous owner already upgrade or are these OEM? 

Thanks!

I would say that yes they have been changed and down graded, buy some joints from trail tough, 20 bucks apiece for Mitsubishi joints or about 52 each for factory, the USA joints from napa and the like will break or wear out the drive line.

In 4th and 5th gear the vehicle feels like it is struggling to accelerate and makes a pretty loud humming noise, is this most likely the rear U-joint? Also, after I replace the U-joint what is the next weak link most likely (it's an 88 Samurai)? I've already done the RRO T-case mount upgrade and replaced the motor mounts which alleviated many vibration issues in the drive line/gear selectors...

Thanks again.

try to get your fingers on the rear pinion nut and see if it is loose, if your u joint on the rear diff was bad you might of loosened the rear pinion nut and now you need to pull the drive shaft and make a tool (angle iron with two holes drilled in it about 3' or longer) to hold the flange and torque the nut I'm guessing to about 150 lbs or so (look it up) and use loctite on it and re stake the nut (hit the notch in with a screw driver like tool and a bfh) and see how that goes.

Cool, thanks. I'm pretty sure I can't create my own tools, but I am familiar with staking from my AR-15 bolt carrier ... But I understand what you are suggesting I do and will have to get with some of my more, uhm, mechanically inclined friends.

by the way sweet ping.