Issue: Cisco 3750-X switch with lan base feature set unable to stack with other 3750G/3750E switch
Solution: the 3750-X switch with lan base feature set will only stack with another 3750-x switch running lan base.
Workaround: install the license set ipbase or ipservices in the 3750-X. you may have to purchase the license. 3750-x switch with ipbase or ipservices whould be able to stack with the other switches. mix of 3750-x,3750G,3750E switches are supported in the stack.
Also, 3750-X switch with universal image comes with ipservices as an evaluation license. you can use the evaluation license as a temporary workaround .
you can verify this using “show license detail”. If the ipservices is listed as an evaluation license you can use the below command to activate it and stack with the other switches. The evaluation license time period is 8 weeks, before that you need to purchase it.
command reference: to set the switch to boot the license
Switch#conf t
switch(config)# licence boot level ?
<ipservices>
<ipbase>
Switch# reboot
< after you install the license you need to reboot the switch . before rebooting please verify the license is activated by using the command “show version”. there you will see that the current license is lanbase and next reboot it will be “ipservices”. if this is not listed like that then the evaluation license is not being activated.
mangattu 
this is very good info. thank you