Hey JD! My wife has a 50 1/2 ton that her grandfather gave her 15 years ago, and hadn't ran in about as long. When I got it going, it had the same problem, only it was also making a noise in first. I took it to Gale here in Joplin, and he told me that the trans was completely worn out. I ended up getting another one for around $125.00 Solved the problem. Yours may need rebuilt, but it wouldn't hurt to have someone take a look at it.
KT


1953 3100 with 1957 235, T5 and 12 bolt 373 rear