Do the aftermarket axles have the retaining clip on the end of the splined part that goes into the differential? The axle should snap into the diff and you should be able to tug on it without it coming out easily. Usually takes a good bit of effort to get them out. Is your machine lifted? That could make a difference too.
Mine: 2018 Sportsman 850SP - 26" Bear Claw HTRs on OEM steel wheels(I really don't like the Aluminum ones)
Polaris Hand warmers, Mirrors.
Hers: 2019 Sportsman 450. 26" Bear Claw HTRs on Moose 427X Wheels.
2007 Sportsman 500 EFI, Polaris Winch, 25" Bear Claws QSC Clutch Kit, Hand Warmers
2001 Magnum 325 MOSE 12" rear wheel conversion. 25" Sedona Coyote tires
2017 Polaris Ranger 1000XP - Roof and front glass
2018 Kawasaki KLR650
2001 Magnum 325 Warn winch and Moose 60" plow-Sent to Daughter and Son-In-Law since his blew up
SOLD 2004 Sportsman 600 Twin