4H pops out

I looked through the last 20 pages of posts before writing this and couldn't find anything exactly like it so I apologize if this question has been asked a million times in the past. Anyway, when driving in 4H, it will intermitently pop out. There is a loud poping noise that sounds like it's comming from the front end somewhere. The 4H light goes out and I can tell that I'm not in 4 wheel drive. It seems to do it more when I'm accelerating and if I let off the gas it will pop back in. If I'm cruising at a constant speed there doesn't seem to be a problem. I thought it was the front axle shift motor so I replaced it and that helped but didn't solve the problem. Looking at the diagram in my Haynes manual I can see that there two check valves in the vacuum lines, could a bad check valve cause this problem? and if so where the heck are they?? Any advice would be appreciated

I think you're on the right track, but you may need to test the vacuum lines themselves for problems, as well as the vacuum switch on the transfer case. The vacuum reservoir, which lives behind the right front bumper, has check valves built into it. I've never had problems with the valves, but the tank itself can crack and leak, as can the lines that run to it, which go beneath the battery and suffer from the acid leaking. A vacuum pump is helpful here, to try to see where the leaks occur.

One useful diagnostic is to check your heater output. The heater controls share the reservoir with the disconnect, so often when there's a leak there, the heater output will default to defrost only under high vacuum conditions, such as cresting a steep hill. If this happens, it's a vacuum leak for sure. If it doesn't, or you've already scouted out all leaks, try the switch on the TC. It's cheap compared to the shift motor.