Is there a difference between the 2005 Jaguar STR vs a base model 2005 S Type rear bumper? my bottom part cracked and was wondering if there was any differences?

 

Pretty sure it's the same, other than a chrome strip instead of body coloured on the STR.
Your old one may be repairable though depending on the damage.
Plenty of choice in the USA but you'd still be lucky to find one in the same colour.

 

All the rear bumpers are the same after the 05MY (Jan 2004) VIN N13089 update, that is apart from the rear parking sensor holes that were not fitted on all models.

Mellow