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It looks as though your caterpillar could be a member of the Sphyngidae (known as hawkmoths here and as sphynxes it the US).
The difficulty is that this web-site is aimed at and largely used by British naturalists so, whilst it is not impossible that someone familiar with the moths of Utah will see your image on iSpot and help you ID it, the chances are not great.
Have you tried the "Buterflies and Moths of North America" webiste? http://www.butterfliesandmoths.org/
It has pictures of many species in adult and larval stages and various identification tools. I believe it is also possible to upload pictures to it for identification (you will need to register for that).
Thank You Justin; I am new to this as I just got a smart phone but I have always known a website like this must be out there. Thanks for the possible identification and the reference. Cheers!
Lat/Lng: 37.147, -113.523