Do you do a lot of weight training Rich? This isn't necessarily bad, but if you haven't done aikido for long your body may be thinking that it takes a lot of force to move something (i.e. your body is reacting to pre-conditioning).
I like to ukemi until I'm kackered - the floor tends to smooth and any tensions in my body. (go forwards, backwards, to the side etc. in quick succesion).
If you don't feel stiff, but people are telling you you are, it may just be the feedback they are getting from your technique.
1. Are you grabbing peoples hands during techniques? Sometimes this is a strong grab which is automatically picked up by uki and causes them to tense. Hold them light enough that you only have to redirect their flow (you may find, as with many people, your index finger detaches from the grab and points in the direction of ki movement).
2. Keep your extension, if your pulling people into yourself too much, uke can feel this as tensing up.
3. remember to keep your shoulders low. If you can't imagine this, do boken cuts, and see how high your shoulders are after the cut.
Alternatively it may be your partner who is causing tension in you (i.e. are they doing the above, or bringing painful techniques on too quickly?).