|Go back into the switch, go into global configuration mode and check the following for me:|
switch trunk ?
Does it show 'encapsulation' as an option? If not, then you can't do that.
I checked on the few 2900 XL's I still have in production and 'encapsulation' is an available option. But on the 2950's (which are newer and have a newer IOS) it's not available. This means the 'dot1q encapsulation' is applied by default when you make an interface a trunk. This might be the case for your switch (ie: encapsulation is applied by default when you make a port a trunk port)