Okay, welded in place normally means it has to be disassembled to remove just the jack from the mounting. Not something I have done myself nor set up to do!
Leaking hydraulic seals are kind of the norm in the field, so I might look for shops who repair hydraulic things on farm or industrial equipment as this is right up their alley at times. If you take a look at a backhoe, it is "normal" for it to be leaving a trail of oil! Grit and bits of rock stuck to the shaft are real killers for the seals, so they know lots more about how to reapir them than the folks at RV shops who may simple replace the whole jack!
No way I can say for sure this will work for your specific jacks as there are certain to be lots of different models made over the years. The problem is that way too many want to chase the bigger profit fixes and not really try to keep your costs down.
So that is where we have to do a bit more legwork to find the guys who do the dirty work every day and understand that it is possible!
If looking for alternates and willing to travel a bit for a better result, it may be worthwhile to consider a drive to factory service?
Certainly not the nice thing to look at but sometimes we do have to get desperate?
But my first idea would be to look to shops who do hydraulic repair, not RV special repairs! Look around for local places that repair or build the lines and they often know who to recommend? Ask the local road crew and they likely know who to call?
Wish you luck on the trek but there has to be somebody fixing that issue.
But not one I know for your area!