Wish I had the answer for the best way to check the front drive axle. I'm one of those that can change oil and tighten a belt but when it gets much past that I come here and ask some of the more mechanically talented members.
For sure I would drive it and shift from 2 to 4-wheel drive. There are times that the 4-wheel drive will bind and it is not that easy to shift even when everything is all right. Backing up can help or lifting the front end with loader will help make the shift also.
For the price, which seems under market value in this area, it may well be worth the $100 or so to have the Kubota dealer check out the whole machine.
As a side note, you mentioned some part time for hire work with the tractor. If you are counting on that to help pay for the machine you may want to check out insurance costs. As soon as you mention backhoe the rates can be a real eye opener and the liabilities pretty big. A cut fiber optic line can cost thousands to repair.
Good luck,
MarkV