A thought, which you might already do. Give her some more lead time to stretch out and such before moving. Maybe pick her up and out, and let her walk around before the climb to the back of the rig and jiggly ride.
Rupert had a good bit of knee pain from arthritis, post knee trouble and operation, plus age. I gave him more time and notice to move as he got old. The guys sometimes had to be made to understand this, as he would hog the passenger seat, and they would sometimes need something from the extended cab behind his seat.