Unusual rumble/vibration on steep incline

[Wingzero_ac19x]

Had kind of an unusual issue since I got my Baja in November. Basically whenever I drive up a steep incline for a while a deep rumble starts and is accompanied by a vibration in the truck. This only happens on steeper grades. The truck has a new transmission and torque converter. Sounds like its coming from the center or rear of the truck. My initial thought was a rear diff issue but Subaru dealer says everything is fine with the truck. This doesnt strike me as normal unless it's something to do with the way the AWD handles steep grades. I'm wondering if it could be axle or driveshaft related maybe having to do with greater strain pushing over 3000 pounds of subaru uphill. But given it occurs after being on the hill for a while, it almost strikes as something heating up. Unfortunately living in the mountains of NC, the main roads here all have steep grades so are basically unavoidable. Anyone have any thoughts or ideas on this?

[redbaja13]

I had a vibe or thumping sound in center floor under acceleration going up hill. Turned out front u-joint in driveshaft was shot.

[Wingzero_ac19x]

Definitely something worth checking out. It will probably be a while before I can get it up in the air to have a look, but I will as soon as possible. Thanks for the reply! I was beginning to think no one would.