replaced prop on older evenrude still no thrust

Hello all I have a 15 hp early 90 model 2 stroke evenrude. Awhile back I got hung up on a bolder at the little red and thinking I was clear tried to take off again. Little did I know the prop was still wedged in the bolder so when I gave it gas it it basically tried to spin while the prop didn't even move. After being told I likely just stripped the prop I went ahead n got a a new one and replaced it. It still does the same thing where it will spin some but not even enough thrust to get off the trailer and sounds high pitched if you give it much gas. I talked to a guy at a marine ship about it and he said it could be stripped gears in the lower unit. Just wanting to get you guys opinions on what all it could be and is this something fairly simply that an experienced diy'er could tackle or is it something that will likely cost me lots of money at a shop. Thanks for any and all help/suggestions

Remove the lower unit.----------------Lightly grip the top of the driveshaft with vicegrip.---------Use something between vicegrip and shaft for protection.------------Turn driveshaft and see if propshaft will spin.------------Shift rod moves up and down to shift it.

Thanks for the tip prolly be tom or Sunday before I can get out there and mess with is...is the lower unit fairly straight forward to remove or are there a bunch of small pieces or anything special I need to pay attention to when removing?