Car Jumps when put in gear

I just recently acquired a 1979 Full Size blazer. I'm excited to get it ready for summer, unfortunatley it's been more of a project than i thought it would be. It's an automatic and when i put it into reverse or drive it jumps forward (or back, depending on the gear) a bit, but definitely more than it should. If anyone has any answers that'd be great. Unfortunately I'm a poor college student so price is a factor here . Thanks!

Is the idle set too high maybe? At first I was thinking a broken motor mount, but I doubt that it would happen in forward AND reverse.

I have a hard time telling because there is no Tach on it. I'm trying to get that worked out though...

does it do it all the time or only at first start up