[screamingchief]

I've been wanting to do mine too. Can this be done on the car lifting most of the weight off the axle and maybe using two floor jacks on the axle tubes like a jig?

Nope.

Anybody that does this any other way other than removing the rear from the car and stripping the housing bare and using a purpose built rear axle alignment jig that locates via hubs in the journals for the differential as the centerpoint for alignment is taking their chances and the results will be a total crapshoot as to whether it stays straight or not.

Do it right or you take your chances,simple as that.

JMO/FWIW.