It's been a few years for me, but I've never seen an off-the-shelf instantaneous angular velocity measurement device.
We tend to drill and tap into casings and then target inductive probes at gear teeth. Using (very) interesting signal proessing techniques, we can resolve angular motion higher than tooth-passing frequency. All you need is some (cheap) inductive probes, a suitable A/D converter and something like Matlab or its clones.
Other methods involve fitting shaft encoders, but (I think) that's over-kill.
- Steve