Hey, Your from my neck of the woods. I grew up in central Idaho just south of McCall. Anyways, I've taken my boat off the trailer several times to work on the bottom. In the g-rage I jack up the back end and then support it with jackstands and padding under the rear corners and intake. Then I use two jacks with boards to support the front as you roll the trailer out a few feet at a time while repositioning the jacks around trailer crossmembers and axles as you go. If you have rafters that would support the front end weight you could strap the front end and just slide the whole trailer out at once.
Just be carefull to spread the load out from jacks/stands with boards as it could be hard on the fiberglass hull, mines aluminum and a little more forgiving. And don't get under it till its well supported with stands after removing the trailer. We lifted boats with a big forklift and 2 straps (1 a couple feet from the transom, the other about 2/3rds the way up the boat from the back) at the marina I used to work at. never had a problem there.
jd