Did polys on mine about 10 years ago. The poly versions went in without a struggle but obviously they cost more than the normal o/e versions. It sounds like your changing the old ones with the back axle dismantled?
If so, try heating the axle housing first the pressing in the o/e bushes, smear some grease on the outer metal casing first then press them in slowly, they should go in straight.
If your doing it with the axle still in place, use a length of threaded rod and washers and slowly force them in that way.