Too short and you use it you can have one end of the shaft flopping around on your pto damaging all manner of things including the tractor
Where to purchase - any ag supply BUT ya gotta know what size the Spline on your pto is. Yup there are different sizes, check the manual.
Pricey? It doesn't matter. Ya gotta have the right size. It's not an option. Around $200 most likely
How to tell if ya do already? Mount the hog, raise it to max height and see if it falls apart, Mark (or measure) it and remove it and see how many inches of engagement you have left, You need about 6" or more. More is better. Then with the tractor and hog on level ground and see if the shaft has some more compression left. It should have a couple inches.
Don't forget to grease the Universal joints AND DON'T OVER DO or you will blow out the Seals.