would not go into gear?

I have a 99 GMC Suburban and recently had to replace the alternator and then the front brakes. After I completed the job the truck would not go into gear. The shifter would move properly but the transmission would not respond. After messing around with it for about 20 minutes turning the key on and off, checking the trans fluid, etc, the truck finally went into gear and drove normally. The problem is this happens without warning and I am stranded at a store someplace. The other day I was driving it and stopped at a stop light, the truck was already in gear but would not move. I had to push it to the side of the road and eventually got it going again. Does anyone have any idea on what I need to replace or repair to fix this issue. I initially thought my transmission was out but now believe I have an electrically problem but don't know how to diagnose. Any help would be greatly appreciated.