I have a MF 1240 4WD...I think this was few models after yours. Anyway, on my front axle there is a bolt with a lock nut to adjust the front axle float. Free play (fore/aft) aft this pivot should be next to 0. My manual describes free play being the thickness of a feeler gauge.
Not sure how that much free play would develop? I spoke to a service manager once about free play, who had lots of experience on the older models. He told me to correct the problem immediately as the shaft could fall out, and destroy the transmission pan, axle mounts, etc. He's seen it before on other tractors and it is big $$ to fix.
Your post is older, hope you can find a fix to your problem.