Jumping out of gear when under load normally means it was not locked in properly in the first case - when moved into low range after a moment or so pull the lever back/forward (as relevant) to make sure it is in, if no joy look to linkage adjustment then if no joy the selectors may need adjustment - open the top of the tfr case and adjust - a relatively easy job.
I had the same issue as you and a tweak of the selectors fixed it. Easy on a 101 as the floor lifts up maybe not so easy on another vehicle.
Garry
REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
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