My farrier suspects my horse needs injections in his hocks based on the last few Farrier appointments we've had. Can someone tell me what the process is for a vet to evaluate this and what the process is if they do decide to inject? What question should I be asking the vet? Are there certain injections that are better than others? How likely it would be a hock issue and not a stifle issue if it comes with the Farrier suspects it from picking up the back hooves to shoe? He's stiff. 
On another note has anyone ever had an issue with hindgut issues causing a back end soreness? How'd you diagnose and treat?

He's a 10 year old trained and seasoned barrel horse. 

thanks!!

Find a good performance horse vet and have them do a lameness exam with xrays of the hocks, a good vet should be able to pin point if it is the hocks or stifles, it may be both. 

The hocks and stifles might both need injecting if the horse has been moving in a way to compensate for pain in either one. Just a thought! Hindgut ulcers can sometimes cause horses hind end soreness if moving in a way to compensate for pain in the belly. If I remember correctly, the soreness tends to be worse on the left side of the horse. Either way, A performance vet should be able to get your horse feeling better.

As far as the hindgut, that can be hard to test for. It might be worth just trying solutions (along with the injections, assuming your horse needs them) such as the stride hindgut program or succeed. If they help then there's your answer! For the the hocks/stifles, your vet will conduct a lameness exam and possibly xrays and then come up with a plan with you.

If you are comfortable giving IM injections and arent already, it might not be a bad plan to talk to your vet about injectable joint supplements - specifically rx ones. I personally would stay far away from the summit brand. Regardless, injectable supplements are much more effective than feed through. 

To test for ulcerpain/hindgut ulcers get some Ritetrack made by ker equine.   That will buffer the hindgut and the foregut. i would use it for 3 days then try him to see if he seems more comfortable.  I do not use this to heal the gut. It is to get by if a horse colics or to use before a show, Or to test one.

I watch my horses when the fairrier is under them. When my stifle boy was hurting he would stretch his hindleg out before yeilding to the farrier and putting his leg into position. he would stretch it straight out behind him then bring it back under him. only for the shoer and not for me to clean, as I never jacked up his hind real tight just to clean. 

also with stifles they tend to look weak during the take off after a turn. they pull with the front end instead of pushing off. also he would lock and do a rollback turn so tight that he came back out through the in door and missed the barrel completely!!!! we figured out he was trying to keep the stifle from slipping. he would get that racehorse jiggle in deep sand round pen as well. 

What state are you in? Someone here might be able to recommend a vet.