I have replaced all U-joints myself and here's my experience:
DIAGNOSIS:
- prematurely you can know upfront which U-joint will fail maybe if you take driveshaft off the vehicle and see if any joint is either too light or too tight to move.
- on vehicle - as told already - you can check if parked in neutral and if you are lucky, you can feel some play if you pull back and forth, left and right both joints. But you need some luck, because it might just have stopped in a position, where needle bearings are aligned right and you will feel no play.
- when you notice "bruuum" vibrations between 40 and 70 mph, its just the high time to replace them. By my experience, U-joints needs some 1000 miles from first slight vibration to the point where it is not driveable beyond 30 mph
I do NOT LIKE U-joints with grease fittings, as they are almost impossible to grease due to lack of space. Also the greasing fitting might get damaged and such joint will die sooner than the closed one. But this is my opinion.
REPLACING DIY:
- quite simple, took me unexperienced some 2 hours
- first off, when you lift the vehicle, take a rag and some alcohol cleaner and wipe driveshaft to be clean for easier handling
- then take a paint or white marker and mark the whole line, from driving axle to mounting axle on diff a line, so you will be able to re-mount in exact the same position afterwards. DO NOT DISREGARD THIS STEP, as you might face annoying vibrations if you put it back differently!
- then if you have pneumatic gun, it's easy to take 4 bolts off. Tap with hammer few times on BOLD/THICK parts of bolt plate to release axle from flange
- take driveshaft off and remove snap rings, which hold bearing caps. MAybe you will need to hammer bearing caps few times inwards to release pressure on snap rings to be able to remove them
- then put driveshaft ON the wide-open massive vise, so the left and right U-joint handles will rest on vice teeth, and the top U-joint will face UP. Do NOT grip it into vice!!! Then hammer the thick part of upper exposed U-joint fork with moderate hammering until joint will slowly loosen up and the U-joint bearing cap will slowly rise through joint fork
Here's the picture, showing how to hold the joint and how to hammer the journal bearings out, but you might need to lean on open vise instead of hand, because journal bearing caps might be quite rusty:
- using same method hammer out all 4 U-joint journal bearing caps until you will be able to take everything apart
- then clean the journal openings with 200 or 300 grit sandpaper slightly, and with small screwdriver clean snap ring seats on journal. Do not sand out too much material, just clean the rust!
- then you can install new U-joint by removing journal bearing caps and CAREFULLY install them with U-joint inside. You can press them together using vise and guide them properly by moving joint half way into one journal bearing cap to guide the other bearing cap properly into seat. DO NOT PUSH THE BEARING CAPS TOO DEEP YET - ONLY TO BE FLUSH WITH YOKE! But be very careful when installing as there are small roller bearings inside caps and if not greased properly, they can fall out and you will have hard time finding them on the floor.
Also MAKE SURE YOU MATCH THE MARKS YOU MADE EARLIER to avoid driveshaft vibrations!
- then you can use either hammer or vise to push all journal bearing caps deeper into yoke, but still NOT TOO DEEP. Maybe half way to snap ring seats.
- then push snap-rings onto bearing caps and snap rings will NOT yet lock into the groove. The will be prepared inside the yournal opening, sitting on bearing caps and waiting for you to push them deeper.
Use vise and properly sized rachet socket to push snaš-rings and bearing caps slowly in, until snap ring will lock into groove.
- most probably you will install U-joint to be a bit TOO TIGHT, so you need to play a minute or two tapping the journal in all directions, until it will move smoothly in all directions, while making sure all snap-rings are in place:
- then just reinstall driveshaft back to the vehicle. I like to match also marks on both sides there, too, so all is installed exactly as it was.