I would most certainly start by checking the transmission fluid. My wifes 1990 300TE wouldn't go into reverse. I assumed it couldn't possibly be the transmission fluid as it's a Mercedes so looked all around the issue until I actually drove the darn thing, it was shifting poorly in forward gear as well, she just didn't notice. It was 3 quarts low on transmission fluid, threw $18 worth of Dexron VI in it and it performs like a champ now. Obviously it's leaking fluid but that was like 6 weeks ago and it's still shifting fine (goes right into reverse) so I am taking the tack of not rebuilding the transmission on a 19 year old car with 124k miles on it if $20 worth of Dexron will work every several months. I'd at least at the fluid level first. Good luck...