1995 chevy silverado 4x4
 

1999 Chevy, Silverado, 4x4 pickup. I think this is electrical but don't know where to start. I am hoping someone else may have had this problem and know how to repair it. I have had it in the shop with no luck. The truck run fine in reverse, but it will not go but 1mph in drive floored. We have noticed that if you crank it up and the PRND light light up on the dash it runs great, but if it doesn't light up it goes 1 mph. any suggestion or help will be greatly appreciated. My mechanic has had it for over 2 weeks now and is still scratching his head. So I am looking to see if anyone else has run into this problem. oh by the way the transmission was fine I had that checked first.
Thanks,:(
Ann