I think somethings wrong

[FAT TONY]

Alright well lately ive been having issues with my truck, first off it only hits 50 mph from first to second it normally does get some speed change but when i hit third gear the speed stays the same as it was in second but the rpms are still going up, i drove her today and she was having trouble taking off, at stop signs i would be in first and it would be barely moving like if i was trying to move it in a higher gear so that worried me a little, i drove it around the block a couple minutes and it started smelling a little bit burnt so i went back home open the hood and checked if the smell was coming from there but it was coming from the back i was looking to see if there was any smoke coming out the tail pipe but nothing then i saw a leak on the floor and noticed oil coming from were my drive shaft connects to the differential , I dont know what that means but i know it is nothing good so if any one could shine any light on my situation id really like to know whats wrong i also noticed there was oil all over my transmission but i did not find a leak any were ill keep looking though