Is it hard to fix a leak coming from my transfer case and can it be expensive?

Take your truck to a repair shop so the source of the leak can be determined and get an estimate from that shop as to the cost to fix the leak! It needs to be seen before anyone can answer your question.

Not a huge thing to have removed and new seals installed. If its the output shaft seal leaking it would not need to be removed to repair. Probaly going to be cheaper to have repaired rather than waiting till it gets low on oil and it totaly fails.

if you remove it its approx 225.00 to reseal the whole case, if you want it done its approx 500.00 job parts and labor

If you don't fix the leak it will cost you a whole lot more for transfer case.