I have been plagued with an unassisted automatic 4 wheel drive engagement at highway speeds (60-75 MPH) on a 1999 GMC Jimmy. The Vacuum Switch has been replaced twice with the Stainless Steel Switch. It now has no engagement of the front axle, the Transfer case engages the front drive shaft but the axle will not engage (Possible Shift Collar Damage). If I try to manually engage by pulling the cable, only a grinding noise occurs. If I try to select 4 Lo, The 4 Hi & 4 Lo lights flash back and forth. Is there a diagnostic I can use to test the Encoder motor and Electronic Control module? I realize I will have to replace part in the Front Differential but, I am tired of being 300 miles from home and having the front axle engage. I have also experienced NO 4X4 in cold-Snowy weather, a reset of the system by disconnecting the battery has restored the 4 wheel drive. Has any one had similar experiences and resolved them? Please share your success or failure with me. This vehicle has been a real POS, I have owned many Chevrolets, one GMC and they seem to share some common failures. My last GM vehicle ever.