I redid the bunks on my portager buy lowering the trailer tongue as far as I could, put some 2x10's or a 4" concrete paving block under the tires to give more 'lift' if you need it, then blocking it up at the transom, (I used two stacks of tires one stack on each side), then raising the tongue as high as I could and blocking the front up. when you let the tongue back down there is plenty of room to work on it. To get it back on the trailer just reverse the process. If you used blocks under the tires, you can take them out and give yourself a bit more room
I've used jacks before with no ill effects, but was never entirely comfortable with it, seems like it puts a LOT of pressure in a small area, if you use them, make sure you place them on a bulkhead or something that provides enough support to the skin.