HI, Vaps. Somebody else might be able to give you some specifics for guitar oriented help, but if it is "pain" type help in general, some weight exercises might help.

I've caused myself some damage of the RSI type to my wrists and shoulders over the years. What improved things a LOT was when I took up a bit of weight training.

If it is stretching and reach, the Spiderlegs exercises on this site might help.

Either way, it's got to be easier than turning ambidexterous. :)