Ujoints are simple to check and change... I've got a few that are the originals with many, many miles on them, then I've got others that I need to change about every 50,000 or so... the wiggle test works... some of the replacements have grease fittings, some don't. I honestly don't think it matters. And yea, they can screw up things. On another car years ago, I had one do some real damage to a rear seal, bearing, and wear the R&P.
Fix the oil leak. If oil can leak out water and dirt can get sucked in. Knuckle seals are pretty simple to change too. The Haynes manual isn't bad. Just like anything else, do one side at a time. I started taking picts at each step on different things... along with wrench sizes and such. (I'm getting old. Memory going and tired of getting up and down for the right size. Plus I owned too many different vehicles.)
Reliability and preventative maintenance-
All this stuff comes down to basically what is in the back of the owners manual, Haynes book, and common sense. If you do water crossings, change all your oils. I extended my vents years ago and don't get water sucked in. I also buy WD40 by the gallon (honest) and spray it on everything. I also buy gear oil in 5 gal buckets. Once you change the oil in both axle housings, trans case, tranny you will start to do the same.