It's really a LOT easier to float the boat off the trailer.
I usually do bunk repairs in the parking lot at the boat ramp. If I show up with everything precut & predrilled, then it's just a nuts & bolts job that I can knock out in a short period of time. My 170 has new bunks, new bunk brackets, a socket set, a ratchet & a breaker bar sitting on top of it now. It's just waiting to be taken to the ramp.
If you must do it at home, I would probably put a strap under the stern of the boat, weaving it through the 2 stern cleats, form a bridal & lift it with an engine hoist that was rented from an auto parts store, then do a similar rig near the bow, then pull the trailer out from under. I would probably do this over grass, rather than concrete if possible. If you lift the stern first, be careful that the tongue jack doesn't collapse on you. I have seen that happen with slightly bigger boats.