82 Cutlass (Cut Dawg) hard shifting, need some help please.

[jasindude1]

Let me give you a little backround on this ride, it was my GF HS car its a custom lowrider, she grew out of that phase and it sat in storage in her dad garage for over 20 years, we just got it back about 3 months ago, she has 34k original miles on her, I was impressed with a new battery and some starter fluid she fired right up, so we r fixing the little things here and there as we find them. Anyways, our issue is when it shifts from Park to R or to D it makes a "thud" noise and jerks the car a bit, after that it runs and drives great, a little low on the idle and sometimes stalls out, but not very often. Any ideas on what would be the cause of this? its a 3.8 v6, I was thinking motor mounts, but very little mileage, the transmission run and shifts great while driving you can hardly notice it all, its just the initial into a moving gear, also after we adjusted the idle a bit it seemed to get worse and louder. Please any help would be great, thank you