Rear leg paralysis typically is caused by one of three things. Too large prey items or impaction or MBD.
Angiehusk did bring up an important point about dragons often lying down with a leg or two stretched out beside the body. However it disturbed the dragon should immediately bring them into a normal position. If she moves around with the let stretched out in an unnatural angle for any amount of time, then its most likely a sign of too big prey items, impaction or MBD.
We will need to know more details of the care the dragon receives now that you own it. How big is his cage? Do you provide UVB light? What is the basking temperature? What substrate do you use (and if you know it, what did the previous owner use?). How big is the dragon (length and weight)? Even though the previous owner indicated the dragon is one or two years old...how well it was cared for really leaves a wide area on what actual size the dragon is.
What have you been offering to the dragon food wise? How much and how often do you feed? Do you use any suppliments? If so, what kind and how often do you dust? When feeding crickets, do you gutload the crickets and what size of crickets (or other insect) are you using? How long has the dragon been in your care?
Answering these questions will make it easier for us to help determine what could be wrong with your dragon. Stiff limbs is a sign of a problem...what that problem is will indicate how severe the condition will be and if it does need immediate vet care. However if the dragon is just stretching out his legs (when held stretched out by choice, the legs are usually parallel to the tail/body or at a slight angle), which he really only will do when sleeping or relaxing on a basking spot or floor of the cage, they won't walk around with their legs held straight, so if he is doing this, then its a sign of a problem.
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PHLdyPayne