96 and 95 are compatible, they use the same wheel, airbags, ignition switch, column shell etc. 91-94 are GM columns, not sure about other specifics, and RENIX ones may work as well since they're GM columns too. 97 through 01 are another split.
putting a 99 into a 91 is not going to happen unless you do dash modifications, wiring modifications, and I've heard you need to modify the hole in the firewall also but not certain if this is true or not.