You can get a free pdf copy of your manual at searspartsdirect. The last digit or two of your model number indicates the series of your machine. For example, I have a 28085 model. The 28 is the deck size, the 08 is the horse power of the engine and the 5 is the series. I am currently restoring a 281019. It is a series 19. Both of these machines can have their lengths changed for belt adjustment purposes but the older one has a clamp under the seat and the newer one uses shims and washers. Suggest you first find the manual for your series and make sure you are loosening the adjusting mechanism. If the tube won’t move, I’d try some penetrating oil on the joint where the tube slides and then pick up the front end and see if you can twist the axle housing to make the tube rotate and break any rust bond that may be holding it in place. Standing it up might make this easier on your back, as would having a person on either end of the axle housing. If the tube rotates a little but you still can’t pull it out, either get a helper or put it in park with parking brake on and attach a come-along or cargo tie down strap to a tree or vehicle and use the ratchet to pull the front end.