Legally, if you did not give it to the neighbor but they simply gave you permission to place it there, it is still your property and you have the right to retrieve it, but would have to also restore anything to the way it had been pre-tub--e.g. cap or remove plumbing you installed, fix any landscaping changed or removed for the tub, etc. If it is economically worthwhile to you to do that--i.e. the value of the tub so exceeds the cost to correct/remediate her yard--then you could sue her to get the tub back; unfortunately, even if you have a legal right to it, only a court can enforce that right for you. Therefore, you would also have to factor in the time/cost of a lawsuit (even you do this "pro se," or as your own attorney, while the monetary cost will be very low, there is a still a time cost--assume you will lose 1 - 2 work days to the legal process) into deciding if legal action is worthwhile.