Turboman
Mab An Gov
+1 on the timber/jacking technique although possibly use 2 jacks, for stability?
I think that is why it is usual to lift the whole tub off. Avoid damage, and possible injury if the tub slips off the jack while you are working underneath.
When I have done it before, we lifted the whole tub, and carried it backwards, placing it on blocks on the ground while we dealt with the chassis issues.