Tranny pops out of reverse under load

[HACKJOB]

I have a l43 that i rebuilt a few years ago in general it runs well but when in 4x4 low in reverse (when in deep mud or sand)  it has a tendency to pop out of gear.  This isn't really a show stopper as i can hold the shifter in when anticipating a "pop out" however i as as i rebuilt this tranny myself, it  is a blow to my mechanical self esteem... can any one pin point the problem  thanks.  

[BigMike]

Only in low range? Excessive drivetrain movement pushing the shifter into the cab or boot - which then pushes it out?