I have a 1995 Silverado Diesel 4X4 with 145K miles. I only drive about once a week to take trash to the dump. It’s been running great until yesterday. I put in drive and went to pull out of my drive way and truck won’t move. It makes a terrible sound in the rear of the truck but won’t budge. I have checked transmission fluid level and appears to be good. I say appears because my drive way is on a steep incline and it’s backed in, bed on the down side of hill. Any ideas would be appreciated.