t case popping out of 4hi?

[74k5]

Ive got a 99 TRD with a manual t-case and a 5 spd and it sporadically pops out of 4 hi while driving. My friend has the same truck with an auto and he does not have this problem. My truck was sitting for about 4 months and it is COLD up here in Alaska. I was curious if the shifter linkage might be frozen up or something. Everything still shifts smooth and the truck only has 50k miles on it so Im hoping that the t-case isnt messed up. If anyone else is experiencing this problem, or knows a possible cause/fix any help would be great. Thanks
oh yeah its also sitting on 35s, I think the gearing is stock (4:10?) if that makes any difference.

 

[KliersLC]

Try adjusting the linkage and checking the fluid levels first and if that doesn't help you'll have to wait on someone who knows that truck better than me.