Greaseable u-joints are weaker since they have holes drilled through the X.Now greaseable joints like CTM's(higher end u-joints) are far stronger as they have a zerk on each cap and no holes drilled through the X.
'02 Liberty sitting on 35" tires,HP44,RockJock60,and AtlasII t-case
Modern sealed u-joints like Spicer's which are triple-sealed are stronger and more durable. I'd only consider a greasable if I only did deep soupy mud and had the energy to keep up with its greasing requirements... but I'd still for strength reasons go with a sealed.
I've had a number of aftermarket driveshafts on my Jeeps and stopped using greasable years ago since they are a PITA to keep up with the greasing. Not to mention, again, that sealed u-joints are stronger than greasable when the grease zerk is in the trunion (body).
The only greasable u-joints that may be stronger are the types with grease zerks on each bearing cap so the body doesn't have to be drilled which weakens it. I do run greasable u-joints on my front axle shafts but those u-joints are CTMs which are about $180/pair which are far stronger than any other u-joint.
For normal applications, I'd stick with sealed. All of my u-joints on both driveshafts are sealed Spicers. Both are stronger and more durable upgrades to the OE Spicers the factory installed.
For your XJ, I'd go with Spicer 5-760x for the front axle shafts and Spicer 5-1310x for your driveshafts, both of which are Spicer's latest "triple sealed" models.