Bang noise when in 4x4

[Stvv]

Its been icy Lately so I decided to start using the 4 high when driving my 87 Xj 4L to work and I am getting a intermittant bang noise(every 10 sec @ 40 mph) from the front end and sometimes no problems at all. This summer I had replaced the front wheel bearings and so I had pulled out the axles and the vaccume shift motor Etc. Afterwards I had tested it out and everything was ok although i didn't really use it since. I wonder if the cold weather(-30C) is causing the vaccume system to leak and that splined collar that links the 2 axles together is then slipping off the end of one of the axles as the vaccum motor retracts?
Any ideas
Thanks -Steve

[gtmud]

Your vacuum motor is not springloaded to release the axle coupler. So I would first pull the vacuum motor assembly off and see, as you removing it if the fork is properly engaged on the coupler and look for any wear. Also look at your switch mounted on the back of your T-case that controls that motor, makesure it is working properly, mine was stuck, keeping my front axle engaged. Look and test your vacuum lines, on my 88 they are metal with a vacuum harness to the motor and the switch. Once your axle is engaged, it should stay engaged untill the vacuum is supplied tot the other side of the diaphram, pulling the couple off the axle.