I am also doing battle with a shop that failed to get work done. It sucks.
Is the shop cooperating, will they sick the German Shepherds on you if you try this? What is the car sitting on? Is the body on the frame with wheels that roll? Avoid trying this with flat tires. Pumped up tires make this job reasonable.
If not hitched to a vehicle, make sure the off-going trailer's wheels are blocked and that the back is supported so it won't flip on you. Back your trailer up end to end if possible. Use both trailers' jacks to try to match the deck heights if possible. Bridge any gap with either trailers' ramps, or you can make ramps from 2x12s with 2x6s screwed on like trusses. Make the ramps as short as you can get away with. Chain or strap the trailers together so they don't part, drop a ramp and bend your baby. Get a come-along - there's a good 8K-rated one from Harbor Freight. Seen those 3" wide, 20 foot long tie-down straps? Home Depot sells them - they make great adjustable pull straps to use with the come-along. Open the bails out to 180 degrees to disengage the ratchets. Pull the strap tight against the come-along, then take up the slack with a few rotations. Pull your baby across. Keep wheel chocks handy to prevent excessive excitement. G-bodies have nice big holes to hook onto in the frame rails aft of the front wheel and ahead of the back wheels. This is where I tned to tie down my cars when I tow them.
This may be over the top already, but if you need more ideas, please reply.
Quinn