Gosh - - my first thought has nothing to due to the loader, but tires....
Oh, I'd check the front axle lube NOW!!!! If it's dry as a bone, you may have found your problem!
You say you really don't know if it was an issue before.......
Your B1550 was made 1988-1997 according to Tractordata.com
http://www.tractordata.com/farm-tractors/001/2/5/1259-kubota-b1550.html
Have tires/wheels, etc, changed over those 20-29 years? Do the TYPES of tires match front/rear? Ag (R1), industrial (R4), or turf (R3)? Somebody stick 16" truck tires on the back?
The rolling circumference ratio of fronts versus rears is very important with a 4WD. If they are out of synch with the tractor's gearing, there can be a bind in the front drive system. For example, a too large front wheel/tire might go too fast compared to the rears and make the front "pull" the rear drive, or conversely one too small might make the rears "push" the fronts. Won't be a problem at all in 2WD.
Same thing will happen if rears are too tall or too short. Type of tire doesn't matter, as long as circumferences work for your tractor.
The key is the ratio between the fronts and rears, whatever kind or type they are.
I'm not a expert - try searching "rolling circumference" on this site. It comes up often on tire type changes.
I'd start there.....and good luck!