If you need a backhoe and the work justifies the cost, then I would just buy a frame mounted backhoe and add it on. Why loose all the money on a trade in (you will loose money no matter how good the deal you think you are getting) when you likely already have the tractor paid for. If it is large enough to suit your needs and the backhoe attachment for it will work for your anticipated jobs, keep it and just outlay the money for the hoe.
The other option is to keep your tractor and buy a used full sized backhoe like a 310 JD or Case or JIC. JD and Case have good reps as being bullet proof. A well used hoe might have a little slop in the pins but that doesn't keep them from working well. As long as the cylinders don't drip oil when used and the engine starts with little to no blue smoke then I would work it. A fairly good used commercial grade of backhoe can be bought for about the price of your backhoe attachment for your tractor.