Before you go and buy new calipers, look the pads and brakes over carefully. More than on guy has posted about brake problems right after putting on new pads, after lots of troubleshooting with no answer, they found they installed the pads incorrectly or they weren't seated on the ledges/rails correctly, making the pad stick.
Pull the calipers and check the pads still slide along the ledges/rails easily and aren't stuck. Also inspect those rails /ledges for gumming up and wear spots that pads fall into. Spots wearing into the rails/ledges that the pads ride on, is a problem for the XJ's, that can grab or stick the pads.
If you can't find anything wrong, then yea, I'm betting on RichP, those calipers probably had corroded pistons that got damaged or stuck when you forced them back down the bore. You could always try pressing the brake pedal with the caliper off the pads (WARNING, that can pop the piston out and make a mess as well tough to put back together) you should feel no resistance in the pedal, if you do, then that piston must be stuck in the bore.